To setup proxmox, you could install it on top of your current debian install : https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_11_Bullseye
Docker in a lxc container is also used quite a lot with proxmox and would allow you to keep some resources without allocating everything for a docker VM.
The bitwarden client caches the database locally, so you can still access your credentials even if your server is down.