GPU over USB if 3rd party drivers can be had is certainly a technical potential (hell PCIe over Ethernet has been a thing for a while) I just don't know if I'd call it a realistic one or one worth using for this use case if it did arrive. Especially since it seems extremely likely the scaled up iterations will be able to do this properly in short order.

I am curious about the other "USB to PCIe cards" though. I know there are NVMe to USB (non TB) adapters these days but I think that's largely due to the NVMe protocol being transport agnostic not that PCIe is actually interacting over the USB connection.

I was under the impression that Thunderbolt and the latest USB protocols actually are PCIe under the hood. This is why you had to have Thunderbolt security, because essentially you can tap deep into the hardware via a simple external port.

Ah yep you're right it supports Thunderbolt 3/USB 4, for some reason I had it in my head the ARM version just had plain USB 3.x type C.

Drivers are an untenable problem. eGPU on regular Macs is helped by there being existing x86 drivers needed for first-party Apple hardware. There’s not much incentive for Apple or the third parties to write drivers targeting Apple silicon for external solutions that aren’t widely used. The cost would far outweigh the income.