I remember watching this project as it was getting started. It was a replacement to all those PHP forums, like PHPBB & Simple Machines BB. This claimed to have more modern features. It’s also open-source, and you can self-host or pay them for hosting. (I recommend the latter)
And since we’re on the subject, has Beehaw considered using an old PHP forum, like I mentioned earlier? They’re really basic and quite nice, IMHO.
i use it to run my forums and it’s significantly better than any experience i’ve had with phpbb. i don’t know how much an activitypub plugin would be actually useful for it, given that that’s basically a niche for a niche, but if all you want is a community forum that gets delineated into different categories and has a fairly robust user-driven tagging system, you could do worse for replacing Lemmy imo
the biggest downside is that it’s not very friendly for low-engineering experience admins, especially if you want it to scale outside of a single computer it’s running on (separating the db from the web traffic, for instance)