Hi Pieteras,
You will most probably find that the following provides the wanted features:
https://pve.proxmox.com/pve-docs/chapter-pvesdn.html
the new beta sdn feature , implement qinq with linux bridge currently. (but if you want to test, it could be great to have feedback)
https://forum.proxmox.com/threads/proxmox-6-2-sdn-beta-test.69655/