I have a few changes I'd like to propose and get feedback on:

New Tier 1:
+1 Prophet and General points per turn. +10 strength for all religious units. +1 trade route from Shrines in cities with neither a Market nor a Lighthouse.

New Tier 2:
Grants Temple of Solomon, which replaces the Temple.
ToS grants +2 Relic Slots (from +1 on the Temple), +25% production toward military units in the city, and Gold bonus equal to the district's Faith adjacency. Inherits all other factors from the Temple (4 Faith, 1 Prophet point, 1 Citizen slot)

Tier 3:
No changes

New Tier 4:
???

This sounds a lot more even in terms of power curve, but I still feel like it could come with a way to generate relics if the relic slot is going to be a part of the Temple of Solomon.

Perhaps all Apostles start with the Martyr promotion? The one where they create a relic if they die in religious combat?

Haven't given it much thought since I posted, but I'm still stuck on my own idea of a Tier 4 promotion. I think it ought to be trade-focused due to how much domination and religion can be so focused on the early game. But then again maybe that means the Tier 4 promotion should double down and give you the tools to really finish off your push to either of those win conditions.

Grant spread charges to religious units and a free promotion level to military units? Grant them each bonuses to strength when one is adjacent to the other?

Since Mont St. Michel gives apostles the Martyr promotion and it looks like the play style wants to keep an apostle near by, maybe have it give the Chaplain ability to all apostles?

Other promotions does seem like the way to go, whether I go with Chaplain or something else.

Sorry for posting here, but a mod I use called Nere's Rebalanced Secret Society adds a mechanic to every Secret Society that adds a Mallus to every Society, for example Owls of Minerva cannot declare surprise war against city-states. Could you add a compatibility in your mods with this one by adding Malus to her when you have this mod active? I even have a suggestion for each of them.

Sorry, but it's unlikely that I will add compatibility for that mod.