Movim v0.28 released within the last 24 hours. It has a web UI (that is optimized for both large & small viewports), E2EE via OMEMO, OTR, or PGP (but users can choose native clients if they wish). With the NLNet funding they are extending to full video conferencing + compatibility with the Dino native GTK client. Subjectively, it looks pretty sharp for a web client. You can also use it to share ‘posts’ for announcements & public feed aggregation—something a group chat should never be used for (announcements & other long-term messages get lost in the black hole search can’t find & unreleated posts all around it with messy-to-follow threads since this sort of content isn’t supposed to be chat).

It’s not quite as easy as services.movim.enable = true for NixOS but the NixOS module isn’t far off once an XMPP server has been selected with optimized defaults beyond standard setup—& the option I would personally recommend for self-hosting as declarative config is easier to work with in the long run, but there are non-Nix options. Being PHP, it’s fairly performant as well as not being built on some space-wasting, RAM-sucking ‘eventual consistency’ model that will cost you out the ass (which is Matrix, by design). The front-end, being mostly vanilla JS, is not using some heavy, bloaty framework. This will meet all your needs & not require expensive hardware host even on an old laptop at home or part of a multi-purpose server (does not need dedicated hardware).