in my last game where i hit lvl 102 (personal best), i had a really small sanctuary build that only used the ss house, the house with the workbench and then i fenced off the street between them so it was a tiny settlement for 8 people. red rocket remained empty and abernathy used the rotc city plan, and once that hit lvl 3, i never went back to it.
that remained as stable as i've ever had it and will be my limits going forward.
in order to preserve stability, i never built at starlight drive-in, i killed the molerats and enabled the settlement, but that was all i did.
every other settlement was enabled and used. it was as stable as i ever got the game - but - this game engine is an odd fish - it behaves as if it's as unique as the end user's machine. so what works for person x may not work for person y - using the same mods, the same amount of ram, perhaps the same gpu...
still, makes it interesting and keeps me off the streets!