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.
PHPBB’s paging system was trash, Discourse’s “infinite scroll” system is only marginally better… from a visual candy point of view; for discoverability, it’s still trash.
Nested threads are a thing since email, and there’s a reason for that. Even Mastodon, a “Twitter-like”, has them. Heck, even Facebook has them (to a degree). The concept could still be improved upon, but going to a non-nested solution would be a serious step back.