Discourse is an excellent choice. Discuss not so much.
The modmore forum also uses Discourse, on a $10/mo digital ocean droplet. Now, that’s not a super active forum, and there’s a little bit of maintenance every month or so to keep it up-to-date, but I’d definitely say it’s one of the best choices in forum software you have today.
The hosted discourse (SaaS) is a fair bit more expensive ($99/mo), but then you don’t have to worry about any maintenance with that. And you probably get some support for that, too.
For something MODX-native, Tickets may also be interesting.