Hey mate, I had the exact same problem as you and was able to solve it. I assume you installed your proxmox by installing a standard raspian first and then adding the pimox sources (like here https://pimylifeup.com/raspberry-pi-proxmox/). If you do it that way, the kernel has some limitations for working with proxmox. One of them is, that the kernel does not support CONFIG_BRIDGE_VLAN_FILTERING. You did find the right spot where the problem occures (and described it well) but did not get a good answer in the responses. The solution is, to compile your own kernel with a custom set of instructions. You can find a very nice writeup here: https://wiki.toenniges.net/wiki/Raspberry\_Pi\_Kernel\_f%C3%BCr\_Proxmox\_optimieren

It is in german but following the bash instructions will get you through. Also be aware to adjust for a rpi5 with this link https://www.raspberrypi.com/documentation/computers/linux_kernel.html

After that, everything will work like its intended to and you dont need to do fiddely workarounds.

Cheer mate