I'd recommend taking a look at the recent D&D Gamma World game, and how they abstract weapons, including guns.

For ranged weapons, you have three choices to make: one or two handed; light or heavy; "gun" or "weapon"

• one handed weapons do less damage and have less range when compared to two handed weapons.
• light weapons are more accurate and use Dex or Int as the attack stat, but do less damage when compared to heavy weapons which use Str or Con.
• "guns" require that you pay attention to ammo, have longer range, and for light weapons increase accuracy, while for heavy weapons increase damage when compared to "weapons"

Ammo for "guns" is handled with a very simple mechanic: Shoot once per encounter, and you never run out. Shoot many times in an encounter, and at the end of the encounter, you're out of ammo and need to find some more.

Non guns are assumed to have recoverable/easily findable/easily makable ammunition, and so you can shoot as often as you like, and never run out.

Examples:

A sling or a hand crossbow could both be light, one handed "weapons": Dex or Int +3 to attack, 1d8 damage, range 5

A tommy gun or a hunting rifle could both be heavy, two handed "guns": Str or Con +2 to attack 2d10 damage, range 20.

Sort Of

This build is legal. However, it probably works slightly differently than you described.

Where might things fall apart?

Shield of Reckoning's trigger is:

Trigger: A foe’s attack against an ally matches the trigger for both your Shield Block reaction and your champion’s reaction.

So in order to use Shield of Reckoning, we need one event that meets both the trigger of Shield Warden (an adjacent ally would take physical damage, per Shield Block), and the trigger of Iron Command (you would take damage from an enemy within 15ft). However, at the moment of Shield of Reckoning's trigger, we haven't Shield Blocked via Shield Warden, so our trigger for Iron Command isn't met.

But it sort of works

The trigger rules remind us that we can only use one reaction (or free action) per trigger. So what caused Shield of Reckoning to fall apart works in our favor here! We first use our Shield Warden + Shield Block reaction to reduce the damage our ally is about to take; we decide to split the damage via Shield of Grace. At that point, our Tyrant is now taking damage; this is a separate event and a separate trigger. So then we can use our Iron Command. This will take two total reactions, but we have more than enough reactions to do that.

But, actually, it could work

The trigger rules also state:

If two triggers are similar, but not identical, the GM determines whether you can use one action in response to each or whether they’re effectively the same thing. Usually, this decision will be based on what’s happening in the narrative.

So your GM can easily decide that, do to the "simultaneous nature of the events in the narrative" (or whatever other rationale they want), that this combo does work with Shield of Reckoning. The good news is: if the GM determines they are separate triggers, we can do it via the two reaction version. If they say it is the same trigger, we can do it via Shield of Reckoning.

Other Build Notes

There are some other potential questions about the build; to show it is legal, I'll discuss them here.

Shield Warden via Bastion Archetype

It is fine for Shield Warden to be taken via the Bastion archetype, and use it to meet prerequisites. There is nothing that says that prerequisites must come from non-archetype feats when taking a class feat. Indeed, when we take an archetype feat in place of a class feat, it is essentially a class feat. From the archetype rules (link is to APG version, but the CRB version has the same text):

Archetype feats you gain in place of a class feat are called archetype class feats.

Additionally, we know from the Additional Feats rules, that when we take a class feat via an archetype, it loses its class trait:

When selected this way, a feat that normally has a class trait doesn't have that class trait.

Oddly, this means that Shield Warden probably has no prerequisites when taken via the Bastion archetype, since both of its prerequisites are bound to a class trait (i.e. the Champion version must have tenets of good and divine ally (shield), while the Fighter version must have Shield Block). This isn't really an issue, since the Bastion archetype itself requires Shield Block, and is a prerequisite for taking Shield Warden via the archetype. I suspect the reason that the Fighter version lists Shield Block as a prerequisite is simply future proofing against possible class archetypes that trade away Shield Block (maybe some very specific offensive fighter archetype or something, or a Psionic fighter). In that case, those fighters would need to grab Shield Block via other means to take their version of Shield Warden.

Be careful where you use your reactions

This isn't really a build problem, but more of a warning: if your GM does allow for the Shield of Reckoning variation to work, then you probably aren't going to be able to use the extra reactions from Divine Reflexes or Quick Block for the combo reaction; Shield of Reckoning is its own reaction. Your GM could hand wave this, but that would create some potential balance issues (since each "extra reaction" feat would be essentially granting two extra reactions).

Is This Balanced?

Yes, for something that only comes online as level 16, I think even the Shield of Reckoning version is balanced. A Paladin Champion could attack the target and have all adjacent allies attack the target while Shield Blocking for their friend, and sooner than level 16 (level 11, I think: take Shield Warden at 6, Shield of Reckoning at 10, then your Exalt ability comes online at 11).

I as a GM would probably allow this, it doesn't seem too powerful, it requires oodles of investment that the aforementioned Paladin Champion doesn't have to do, and probably does less damage overall, despite coming online at higher levels.