I use Webcalendar - https://www.k5n.us/webcalendar/

Its been a while since I’ve looked at the alternatives, but this has served me well for >7 years

I have Dovecot and Postfix running on Debian on a server in my closet. Works great for my needs

I use Debian and Ganeti. I’ve not toyed with much else, so I dont know how they compare.

I just use a few shell scripts. Basically some simple bash scripts that do all the work after a fresh install.

Kubernetes/Ansible are great to learn, but are also more complicated for simple setups.