Interesting. I tried to use mobilizon which I think is activitypub-based but has somehow similar goals as Karrot. Karrot looks a bit lighter/snappier to use, which is good (but TBF it’s been a while since I haven’t given Mobilizon a try, maybe it got better since last time I tried).
Matrix tries to kill XMPP but the reality is that if you want to self-host, XMPP is much less of a hassle. Also, Matrix is an open standard as in “pay big money to participate in the openness”. https://matrix.org/blog/2022/12/01/funding-matrix-via-the-matrix-org-foundation/