Hi Christian,
This arm64 board [1] with a x16 slot was announced recently and is $450 for a 64GB version. Though I wouldn't buy it until it can run a vanilla Debian (it may already, I haven't checked). Just for confirmation that they are committed to upstreaming necessary changes.
Alex Voicu acquired one of those Radxa Orion O6 boards [2] and tested Debian's ROCm packages on it. He reports that the tests nearly all pass, although there may be some limitations (e.g., there were some difficulties with Large BAR for GPUs with >8GB of VRAM). If I can get my hands on a 64GB Radxa Orion O6 board, I'd like to watch out for any regressions in ROCm on ARM by adding it it to ci-test.rocm.debian.net.
Sincerely, Cory Bloor
[2]: https://radxa.com/products/orion/o6/[1]: https://www.tomshardware.com/pc-components/motherboards/worlds-first-open-source-armv9-motherboard-surfaces-radxa-orion-o6s-pricing-starts-at-usd200-for-the-8gb-ram-model