Oh wow congrats, I’m currently in the struggle of stretching an ab350m to accept a 4600G and failing.
You’re right, you should hit PCIe 3 speeds and it’s weird, but the fact that the drives swap speeds depending on how they’re plugged in points to either drivers or the chipset.
It’s a gigabyte ab350m gaming-3 rev 1.0. it boots grub fine but then crashes right after displaying “loading Linux 6.x”, CPU led flashes then dram led stays on, I have to turn it off with the PSU switch.
Either it’s a rev 1.0 bug which is a thing on those motherboards, or the CPU (or igpu) is defective.
https://superuser.com/questions/1854228/proxmox-doesnt-boot-after-cpu-change
I’m currently waiting on support from both the seller and gigabyte but I don’t expect anything out of it, though I’m still yet to test it in a different motherboard.