I can recommend you Debian, since it’s the “default” for many servers and has a lot of documentation and an extremely big userbase.
For web interfaces, I can recommend you, as you already mentioned, CasaOS and Cockpit.
I used CasaOS in the beginning and liked it, but nowadays, I mostly use Cockpit, where I have the feeling that it integrates the host system more, and allows me to do most of my maintenance (updating, etc.) quite easily.
CasaOS is more aesthetic imo, and allows you to install docker containers graphically, which is better for beginners.
I personally do my docker stuff mostly via CLI (docker compose file) nowadays, because I find it more straightforward, but the configuration CasaOS offers is easier to understand and has nice defaults
CasaOS isn’t an OS, it’s just the web interface you install afterwards you have Debian or whatever running