I guess I'll postpone the forum changes a bit for now and focus on other new software. Ultimately I am hoping that in a few months the fully remade and responsive phpBB theme will be out and than that would be a good time to reinstall a phpBB board. I like that phpBB works better without Javascript (the new theme likely as well) and that it has XMPP notification support.
Flarum8 is nice, but still a bit buggy, which I why I removed it again for now. I also understood that many people here didn't like the Javascript only nature of it.
Ideally there was a Activitypub compatible forum software or a nicer forum view for the group boards in the Hubzilla