Nextcloud is reall nice but if you are using many of its functions,. For just calendar it seem a bit to heavy, isn't it?

I once was fixing someones computer booting with Bluescreen, because Windows 7 thought it found newer drivers for USB 3.1, and those newer were causing BSOD

modern anti cheats work under linux right now, from what I know

That probably depends on your vps provider (if using vps). Hosting exit node at home is clearly a bad idea.
Check this (mind that this not have to fresh enough): https://community.torproject.org/relay/community-resources/good-bad-isps/

Damn, seeing latest commits to branch I guess it it time for another update of my instance.

Yeah, currently it is bad time for computing power donations.

I think I need to do something similar with my cloud. It seem good both for organizing my cloud and for plain visual organizing training.

My config files are my documentation, but what is not suitable to be written in config files I write in selfhosted BookStack

I would not recommend docker-compose for a begginer. As first, one should learn basics, then optionally switch to docker-compose to automate stuff he already know. Also bind mount volumes are a better solution for long term storage than default volumes, since docker will never delete those, and their path in host system is configurable.

Stable release seem a really good idea to help bigger instances keep stable, as that is what users probably want. Meanwhile mine likely will follow devel branch :]

Unseen

using apt for managing all system packages seem like a security flaw. If it get corrupted or run badly, whole OS can be destroyed.