Using an online Challenge calculator, I am not seeing the issue that you are describing.
Monsters exist up to CR 24, and then the Tarrasque by itself is 30.
For 4 Level 20 PCs, a single enemy of CR 23 is at the high end of a Hard encounter, just barely below the threshold for a Deadly encounter. A CR 24 is thus a Deadly encounter, but 5 Level 20 PCs push it back down to a Hard. (The Tarrasque requires 7 Level 20 PCs to push it under the Deadly level, but lets ignore that one! Using that, you are definitely threatening a TPK!)
I believe you are calculating challenge improperly. I will explain what I think you are seeing, and you can obviously correct me if I am off base.
For the CR calculations(using the table on page 82 of the DMG or page 56 of the Basic DM Rules), a level 20 PC has a Deadly threshold of 12700 XP. A single CR 15 monster is worth 13000 XP. Since that is higher, you see it as being a Deadly encounter. However, for encounter calculations, you must add in the XP threshold for each party member to arrive at your overall thresholds (which is explained in step 2 of the Evaluating Encounter Difficulty section on the same page as the XP Chart). For a party of 4 Level 20 PCs, the Deadly level is set at 50800 XP. A single CR 15 monster against a party of 4 Level 20 PCs will be an Easy encounter.
This is not unusual; Dungeons & Dragons has a long history of giving the heavens and their denizens fairly minimal treatment. They just aren’t as interesting—they get along, they generally support the players (who are generally at least non-evil), and so on.
Nonetheless, previous editions have discussed, at least in broad strokes, the leadership of these planes. Detailed breakdowns about each member, and their individual agendas, however, are rarer. Again, they kind of tend to get along; there is just less scheming for individual priorities going on in the Upper Planes. Not saying none, certainly, but much less.
Anyway, here’s the broad strokes, per the more-or-less consistent Dungeons & Dragons cosmology that we’ve had since at least 2nd edition. Much of this is only briefly mentioned, and sometimes simplified, in 5th edition, but I treat those simplifications as being solely in the interests of keeping descriptions brief, rather than a strong statement that things have changed.
Exemplars—the true expression and rulers of the outer planes
First of all, the archdukes of Hell are devils, and the demon lords and princes are, as the name suggests, demons. This makes them the exemplars of their alignment (LE for devils, CE for demons), and of the outer plane of that alignment. See here for more on the exemplars. Exemplars are literally an alignment-incarnate, and they are formed from the substance of the corresponding outer plane—which is literally formed from the belief in and adherence to that alignment. So devils are formed from Hell, which is made up of solid lawful-evil-ness, and likewise demons from the Abyss as chaotic-evil-incarnate.
The upper planes have exemplars too, and like the devils and demons, they have their own governing structure:
Mount Celestia and its archons are ruled over by the the Celestial Hebdomad. The leader of the Hebdomad is Zaphkiel, the only remaining member of the original seven martyrs that created the seven heavens. The other six tome archons on the Hebdomad are Barachiel, Domiel, Erathaol, Pistis, Raziel, and Sealtiel.
Elysium and its guardinals are ruled over by Lord Talisid and the Five Companions (or Five Champions), Duke Lucan, Duchess Callisto, Duke Windheir, Lord Hwyn, and Lord Rhanok.
Arborea and its eladrin are ruled over by the Court of Stars, headed by Morwel, Queen of Stars, and her consort Faerinaal and champion Gwynharwyf. Other members of the Court do not appear to have ever been given names, or even counted.
Other notable planar residents
These are not necessarily the only institutions of power and influence in the heavens, just the official governing structures of those planes—the counterparts to the archdukes and demon lords/princes. For example, the Seelie Court of the fey are good, and found on the Upper Planes—but their counterparts are the fey of the Unseelie Court, not the archdukes or demon princes. The fey control their own realms within the planes but do not control entire planes the way exemplars do.
Gods and their relationship with the planes and exemplars
Another important example are the good gods and the angels. Deities tend to have their own realms in the planes, including good gods in the heavens, but those realms are separate from the rest of the plane as a whole. Gods are, effectively, guests in these planes—the exemplars enjoy a much closer relationship to the plane itself than the gods living there do.1 This special relationship makes it very difficult to attempt to usurp them, and the strings attached to god-hood largely prevent deities from trying to do so. So deities keep themselves apart from the governance of that realm as a whole. They certainly can interact with exemplars, may have exemplars among their worshipers or agents, can act against individual exemplars in the pursuit of their own interests—but they don’t get to interfere with the ruling of the plane itself. The plane, and their own god-hood, won’t let them.
In short, while good deities are important, powerful, and influential residents of the upper planes, they are not part of the governing structure of any of those planes. That means they are not analogous to archdukes and demon lords, and so are an inaccurate answer to this question.
As a notable almost-exception-that-proves-the-rule, the Hebdomad has four gods (Bahamut, Heironeous, Moradin, and Yondalla) who sit on it, alongside the seven archons. However, those gods take an advisory role, and defer to the rulings of the archons, especially on matters relating directly to archons or Mount Celestia.
Last but not least, some sources refer to the leaders of exemplar hierarchies as gods, but this is an oversimplification. They can be, and are, worshiped like gods, and their faithful can receive divine spells and other blessings through that worship, but at least in more detailed coverage of the outer planes, a distinction between exemplars—even very powerful ones—and gods is important. Both positions have great, cosmic power, and both positions come with some strings attached, but deities receive both greater individual power as well as narrower concerns and constraints than exemplars do. This fact is particularly important to Asmodeus, the Lord of the Ninth, who leverages the fact that he is not a deity, and thus freer to act, to the hilt. The Lady of Pain in Sigil is also exceedingly adamant about not being a deity—but what, exactly, she is, is a matter of pure speculation (but I wouldn’t recommend speculating about it too loudly).
Angels
Finally, angels (devas, planetars, and solars) are special, powerful, servants of the gods. Unlike true exemplars, angels can be any good alignment, rather than being the pure expression of exactly one alignment. Being directly related to good gods, they too stay out of the affairs of the Hebdomad, Five Companions, and Court of Stars, and so again are not a good answer to this question. Notably, other alignments lack a counterpart to angels.
Elementals and the inner planes
The inner planes are different from the outer planes; they are matter and energy rather than faith and belief. As such, elementals are expressions of stuff rather than alignment, which means they aren’t quite the same as the exemplars of the outer planes. They tend to be neutral just because they are uninterested in aligned matters, unlike say the rilmani who are the exemplars of truly neutral alignment and care quite a lot about balance—and elementals can be whatever alignment suits them (technically exemplars can too, but it’s vastly harder for them). Nonetheless, elementals are similarly “the plane incarnate,” so there are also similarities. And there are the archomentals, four very powerful, good-aligned elemental lords, one for each element. Their counterparts, though, would be the princes of elemental evil, rather than any devils or demons, making them similar to the fey as being not quite a correct answer to this question.
The positive energy plane and negative energy plane are nearly empty, and their few denizens do not form societies of any description.
- With the notable exception of Lolth, who is both a demon prince (styling herself Demon Queen though that’s just her personal title) and an evil goddess—the only individual I’m aware of that does both like that. Otherwise, exemplars don’t take kindly to gods thinking that their deity status grants them any say in planar governance. Lolth only gets around that because she had lost her deity status and became a demon, and then a demon prince, and only subsequently regained her god-hood. Well that, and because no one really wants to mess with Lolth.
Best Answer
This answer is based on 3rd edition material, and I don't know whether there's anything in later editions that contradicts this.
The 3rd Ed book "Fiendish Codex 1: Hordes of the Abyss" has some material about what happens to demons slain on the Material Plane. The tl;dr is:
Quoting from pages 7-9 of HotA:
By my understanding, there's no official dimensional anchor in 5th Ed, but you could take this as precedent for the idea that magic can preserve demonic corpses, and perhaps persuade your GM to let you research an appropriate spell or item. Dimensional shackles or a modified version thereof would be a likely option.
The book then gives a "Demonic death throes" table, with a d20's worth of suggestions for what might happen when a demon dies outside the Abyss (excluding those like balor which already have a specified death effect). Some sample options:
Most of the twenty examples involve total or near-total destruction/annihilation of the body, and most of these happen within seconds, so you wouldn't have time to mithril-plate anything. But there are about three in the list which look like they might allow for salvaging bones.
So, you will probably need to ask your DM about this, and either kill demons until you find one that dies the right way or research some magic that lets you preserve the body. But perhaps that material will give a starting point for those discussions.