I tried bringing up a Radeon R7 250 card which does highlight a number of limitations with the pcie interface on the RK3588. Anyway some things do work and a write up on my blog.
Adventures with external GPU card + Gen 3 pcie
What is the status now?
Also, I guess main thread for this topic is currently https://forum.radxa.com/t/idea-about-plug-amd-gpu-via-nvme/13132/23.