I'm absolutely certain that cavern layers will join together. I usually gen 1 cavern worlds for FPS reasons, and got all the cavern stuff in that one. I've been doing 2 layers now though, since I realized that uprights only spawn with 2 or 3 layers. Everything is present and will join together into one super-cavern.
We do NOT need reactions to produce free materials. This is not a "lol let's godmode some walls" project. If we wanted easy mode to produce forts, we wouldn't be playing Dwarf Fortress. No. If a player wants gold, they better get digging. Players are using local stone or they're importing. We are also NOT hacking in free expensive items for loot. If gold goblets become common in dungeons then what's the point of carrying them at all? The only ONLY justifiable free resource would be the heat-sensitive stone used to make things deconstruct in theatrical ways. The only Only ONLY justifiable use of DFHack to place liquids (or, for that matter, the majority of DFHack's utilities) would be to theatrically deconstruct items. Things like autodumping and especially cheap shit like runesmith genocide is NOT welcome.
This is NOT a sandbox, this IS constructing many, real, hard fortresses in a single world. We're not just doing this for the hell of watching walls go up. That also means no speed:0 or no_eat or anything.
I'd also suggest a few tweaks to domestic animals, like changing turkey to [CHILD:2]. This doesn't change their growth rate, but it does make it so that "the poult has become a gobbler" message will appear when the animal is actually at adult size and ready to butcher. Default has the turkey becoming an adult at 1 year, but fully grown at 2 years, and butchering becomes tricky. Instead, have it announce their full size for ease of butchering. Similar for many birds and other relevant animals.