I'll be honest man, just don't do it. I tried, I really did, to make this exact scenario to work. You can get it to work - but it will be extremely brittle. You're essentially hacking around LXC to do things it wasn't built to do, and most of it is disabling security that's there for a reason. At the end of the day you are essentially running docker directly on the host anyway, the passthrough lxc becomes less and less "there" vs passing stuff through. Then, every update to proxmox became anxiety riddled because every update would change or break something on my setup.

If you want to continue, more power to you, but I hope you heed my warnings. This is a path you will spend a lot of time on and experience a lot of frustration. Spin up a tiny debian VM and run the containers there, the overhead of the VM has been negligible, and any speedup I might have had has been made up 10x by cutting the amount of time I've had to hack proxmox to make it work.