So maybe some sort of island / underground testing facility that is like a huge city or something, something isolated so no one can do the whole; "Well I just call for help." Sorta thing. The mish mash of different monster types would be like in key locations like nests and stuff around the city but like the Zombies would be everywhere and have like maybe one or two types. Like RE zombies which over time turn into crimson heads and lickers, Dead by Daylight type zombos, and then like Last of Us spore parasite like zombos?

The plot would follow more of a resident evil type thing so like; You know next to nothing it's a simple; you wake up and suddenly zombos. And if you'd want you can try and figure out over time what the hell is going on and if people make discoveries and find stuff out IC then it pops up in the lore/site wide plot.

(This is a crazy idea but...)
As for what type of forum provider... crazy idea but... why not both? Like have a proboards and Jcink site which are linked through a common discord, the site information and layout is all the same and the staff on each site keep each other up to date about what's going on and try and synergize the two sites. Incentivize being on both sites, give players an ability to port over characters from one site to another.