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?

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.

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.