You also need to discuss expectations if you haven't already
If you just change the mechanics to better fit the style of play that a highly lethal old-school module is written for, you need to discuss the expectations of this style of play with your players too. If you don't talk about these expectations, all that your players will see is that you've changed the system and made them less powerful apparently for no reason. I would highly recommend having your players read a quick primer on old school play; if you can get past the bias inherent in the document, it's very helpful. Even if you don't have your players read the primer, you can still state the main points and present the reasoning behind you've changed the system. Making sure players know that build is less important, that death is frequent due to lack of caution, and that player skill is rewarded is going to be necessary to support the style of play you want here. Other than this...
Running an LotFP module with 5e is perfectly viable with some conversion work
As long as you convert the module's mechanics over, running Tower of the Stargazer should work fine. The main issues would be hit point inflation (which is sounds like you've covered); the Perception and Investigation skills (which it also sounds like you've removed), and Saving Throw DCs. I'm assuming (based on your question) that you feel comfortable making mechanical changes to both the system and the module, so I don't think this will be a problem for you. One point of caution I'd like to advise on, however, is length of time spent to make a character.
Avoid Lengthy Character Generation
The general expectation of a game like LoTFP is that characters die frequently, and thus, must be created frequently. You'll want to keep the time a player spends out of play due to a dead character as low as possible. A LoTFP character can be created somewhat more quickly than a 5E character, so I'd advise making some of your system changes reflect this. Particularly; I'd say that you should have a list of cantrips and 1st level spells available as handouts for your spellcasting characters so selection goes more quickly, that you either enforce rolled stats or use a standard array for scores to not spend a lot of time on point buy, and that you don't use feats and skills (which it sounds like you've already done). This is just a very quick and dirty method of reducing time it takes to create a new 5e character, but hopefully it will work for you.
In Unearthed Arcana: That Old Black Magic (2015), they playtested a Tiefling Variant with an Abyssal Tiefling subrace. The Abyssal Tiefling gets this Abyssal Arcana trait instead of the original Infernal Legacy trait:
Abyssal Arcana. Each time you finish a long rest, you gain the ability to cast cantrips and spells randomly determined from a short
list. At 1st level, you can cast a cantrip. When you reach 3rd level,
you can also cast a 1st-level spell. At 5th level, you can cast a
2nd-level spell.
You can cast a spell gained from this trait only once until you
complete your next long rest. You can cast a cantrip gained from this
trait at will, as normal. For 1st-level spells whose effect changes if
cast using a spell slot of 2nd level or higher, you cast the spell as
if using a 2nd-level slot. Spells of 2nd level are cast as if using a
2nd-level slot.
At the end of each long rest, you lose the cantrips and spells previously granted by this feature, even if you did not cast them. You replace those cantrips and spells by rolling for new ones on the Abyssal Arcana Spells table. Roll separately for each cantrip and spell. If you roll the same spell or cantrip you gained at the end of your previous long rest, roll again until you get a different result.
Abyssal Arcana Spells
| d6 |
1st Level |
3rd Level |
5th Level |
| 1 |
Dancing lights |
Burning hands |
Alter self |
| 2 |
True strike |
Charm person |
Darkness |
| 3 |
Light |
Magic missile |
Invisibility |
| 4 |
Message |
Cure wounds |
Levitate |
| 5 |
Spare the dying |
Tasha’s hideous laughter |
Mirror image |
| 6 |
Prestidigitation |
Thunderwave |
Spider climb |
I would assume that homebrew could trust this list to be at least somewhat balanced as possible spell replacements, since it made it to UA. However, keep in mind that UA is not strictly canon, but rather playtest material from WotC game designers.
In that regard, the Infernal Legacy trait looks a lot like a Dragonmark (from Unearthed Arcana: Eberron (v1.1)) or the Magic Initiate feat (PHB, p. 168). You could also skin one of those feats into a racial ability, as a homebrew that wouldn't deviate too much from the rules.
And completely RAW, you could always do a Feral Tiefling with the Devil's Tongue trait (SCAG, p. 118), too.
Best Answer
Multiple of the same proficiency does nothing
Having two features which tell you that you have proficiency in a given saving throw (or any other roll), you still only have proficiency in the roll and add it once. That Keldon trait thus does nothing if your class already has proficiency in the saving throw (you can ask your DM about replacing the saving throw proficiency with another saving throw, their call. They may want to limit you to gaining proficiency in Intelligence or Charisma instead, as those are generally considered similar to Strength in their usefulness. Dexterity, Wisdom or Constitution saves come up more frequently and are thus generally better).
The relevant rule preventing proficiency duplication from doing anything is found both in chapter 1 (Proficiency bonus section of step 2) and 7 (under Proficiency bonus), beyond proficiency being defined as a boolean (so you can only be or not be proficient) reads:
It may be worth noting that saving throws are not included in the optional rule for changing the proficiencies granted by your race in Tasha's Cauldron of Everything (See Customizing your Origin > Proficiencies). This is presumably due to none of the official race options granting saving throw proficiencies.