After noticing in the configs that Geologica lapis ore doesn't generate when GregTech is installed, and the COG version of lazurite veins wouldn't generate since I had disabled GregTech ores in the COG tabs, I went ahead and installed Mystcraft and explored a random age for a few kilometers, but then it lagged so bad that I tried to quit the game and it got stuck on "Shutting down internal server" for 10 minutes and I had to use MultiMC's "Kill Minecraft" button. In the morning, I'll see if the game will load.
Edit: the world loaded, but then quickly lagged completely again (after killing two cows, the third showed no reaction to being attacked, though I was able to travel further north to the edge of the generated chunks, and I even tried to use my linking book, but the GUI for it wouldn't open)