The Orange Pi 6 Plus is full UEFI and boots things like FreeBSD natively. It's a bit higher end than a Raspberry Pi, but they're definitely in the SBC market. I'd assume they'll work with Proxmox if they support the newish 5G ethernet chips out of the box.
That thing is apparently $366 USD with 16GB of RAM, so my first thought is really "it had damn well better boot things properly if it costs so much".