what might everyday people use to set up forums as relatively easily and cheaply as their Discord servers, and not have them riddled with ads or other clunky elements?

Discourse is a clean open source forum software that is commonly used for application support and well suited for it.

Or if your a real die hard for the fediverse, you could set up a lemmy instance for application support. There’s even a phpBB frontend for an oldschool forum look and feel for it.

Usually everyday people don’t setup forums, that’s the responsibility of the application owner(s) or provider. In this case, the easy option is also the shitty option if measured by discoverability of the content.