In my experience kagi is decent, definitely gives more forum results than google. I've found yandex to be the best at finding all kinds of forum /discussion sites.

There is a seperate "lens" (think like "images", "videos" and "news" in Google, only there are more and you can create your own) for "Small Web" which only includes what they describe as "Results that favor noncommercial domains and topics".

(Other standard lenses include

- Forums

- Fediverse forums

- Usenet/archive

and I think 7 others.)

Try search.marginalia.nu

From the website about: "This is an independent DIY search engine that focuses on non-commercial content, and attempts to show you sites you perhaps weren't aware of in favor of the sort of sites you probably already knew existed."