There are a few I'd recommend:

Loomio - this is usually for coops, especially decision making, but last I checked works well as a forum.

Lemmy - federated reddit alternative.

Discourse - the forum we know and love.

Flarum - decent alternative to discourse.

The challenge with all these is moderation: Lemmy solves it best by having subreddit style division of labour, with moderation per "board". Discourse supports trusted users if memory serves, and I'm not sure about the others.

I'm pretty sure discourse and Lemmy also support eg, log in with google/facebook/etc which eases onboarding a lot.

Personally, I'd go with Lemmy. It is less mature than discourse but probably more suited to your purposes.