Well, if it's your first time selfhosting and setting up your environment, I'd to things in steps. Great thing about virtualization is that you can snapshot.
Once you get proxmox up and running, choose an OS to virtualize, snapshot once made. Snapshot once configuration for users etc, snapshot once docker has been installed, snapshot after docker-compose has been configured.. You get the idea.
Set up backup of the VMs/containers, set up lemmy using the official docker-compose step-by-step guide. Once you reach it on port 80, set up nginx.
They can look all they want, but someone needs to stop these big guys from monopolizing the whole industry..