Complaints:
Buddhism didn't found automatically at 300 BC in my first 1.12 game. This could be a fluke, but has anyone else experienced this?
Units that you don't have the resource to build should be available under the mercenaries civic. They should just start with the mercenary promotion. Otherwise, you're removing a lot of the flexibility for civs that don't have iron or bronze (read: Phoenicia).
The dead civ culture mechanic is utterly broken. I have two plus unhappiness from the motherland penalty in Babylon, and it's aggravating to measure culture at all because of the bugs. When I hovered over the surrounding tiles, they were 100% Persian. Going into the city screen it claimed a proportion of 56% Persian culture, however, the colors on the bar showed about 80% Babylonian culture (which was probably accurate). A few turns later, and the numbers changed to 82% Persian culture, an inverse of the reality.
Look, there might be some potential for keeping dead cultures alive. But this is emphatically not how it should be done. It's not like it even affects respawns.