You would have 12 GB ram shared over all the dockers. I think you will be fine. Unless everything will be used intensively continuously. But that’s my opinion. Just give it a shot, nothing to loose. Promox itself does not take so much. So if it it does not run in this setup you need different hardware anyhow.
I don’t like the solution of running docker next to proxmox, not in a vm, you want proxmox to respond even if the docker vm is busy/overloaded.
In terms of backup you should be good. I would skip that weekly local backup construction, not sure what that adds if the off site backup is working reliable. I’ld format that one and add proxmox to it and make proper use of it (like a second docker vm)