Also, there is already a great mod in the workshop that adds plagues to the tier 5 Pestilens building (Plagues for the Plague Cauldron) that uses scripts to make it only work if the building belongs to the pestilens faction; It's fully compatible with this mod and because it uses scripts (which I am not familiar with) it's a way better mod than I could ever hope to make it.

This mod is not SFO compatible, only the main mod is (because somebody bothered to port it, link to the SFO version should be on that mod, but that doesn't change the skill tree)

I don't make any SFO compatible mods myself because I am busy working on other mods and have never used SFO myself; if it's not compatible that's probably because SFO replaces the recruitment buildings; it's probably quite easy to make a compatible version, but I am not going to release an SFO version because I don't know SFO is balanced; I don't want to release a mod that I have no way of knowing if it's properly functioning and balanced.

Vanilla - no bugs
Radius - no bugs
SFO - the catapult does not move

Indeed, the Pestelens building needs to be made more expensive. And the Scryer building is cheaper. You can also add a plague to Tier-5 Pestilens building.