Holy Guardian Fallen Empires seem to no longer spawn as many Sacred Gaia worlds as before, or sometimes, they are within their own borders. Re-checked through more playthroughs and in observer mode, as pre-Heinlein there certainly were about 5-6 just outside their borders for the 600 size galaxy. I currently barely find ~3, or definetly way fewer than before. All in all they're nowhere near as much of a blocker as they were, or as the Isolationists are. They also don't mind that you terraform then settle on a Tomb world, opposed to getting triggered when simply settling on a Tomb World.
Additionally the habitable planet slider doesn't seem to completely work as intended. I cranked it to 200, then all the way to 500% and they don't feel like they're 5 times as many planets. I can't fully confirm this one though, since perhaps with habitability changes the right planets -do- feel like they're fewer with the 2 extra types. And in all fairness, there are systems with 4 planets and horrifyingly many Tomb Worlds too! I guess the Great Filters were hard at work!