Quarto has theming via several built in bootstrap themes.
Quarto is written in javascript.
Also, it has no template engine/templating. I have a nasty hack where I write python code blocks (vua quarto's ability to execute code blocks) to output markdown that can be remdered to both html and pdf, which is pretty unique. But this is probably not what most people making a website want.
You may want quarto. It is explicitly designed for data science and related usecases, and has ample support for the graphs those usecases need.
I use quarto: https://quarto.org/
The big thing I like is that it has fulltext local search, built in and easy to enable.
Search is possible on hugo, but it's not built in, you have to get a plugin, etc etc. Same for many other options you mentioned.
Heres my website: https://moonpiedumplings.github.io/
You want either mattermost or the whole matrix stack (backend, plus element with voice/video calls).
Matrix/Element is more of a discord alternative, whereas mattermost tries to be more of a slack alternative, where it seems to have some calendar integrations.
Big bluebutton is now integrated into Canvas, an open source learning management software (LMS) that every school I have went to has used.
Here's my older post on a similar topic on reddit: https://www.reddit.com/r/AndroidGaming/comments/g0sw0k/i_am_making_a_list_of_source_ports_on_android/
Yeah, creative writing communities like r/AmITheAsshole, r/relationshipadvice, or r/offmychest.
PostmarketOS is switching to systemd