Storage Spaces Direct HDD MediaType is unspecified

You have to have supported media types with Storage Spaces Direct which is no-RAID, no-FC and no-iSCSI. Should be SATA, SAS and NVMe.

https://docs.microsoft.com/en-us/windows-server/storage/storage-spaces/storage-spaces-direct-hardware-requirements

Drives

Use local-attached SATA, SAS, or NVMe drives.

You can easily workaround this issue if you say don't have compatible SAS HBA and want to use RAID controller and single driver RAID0 nodes by forcing RAID -> SAS using registry tweaking or installing third-party filter driver to do that.

https://www.starwindsoftware.com/blog/resolving-enable-clusters2d-bus-type-support-issue-on-some-storage-controllers

I can't find the driver though...

P.S. In general I'd skip using S2D on only two nodes, it's clumsy at best: fault tolerance isn't great: cluster can't survive second disk or node crash in a row, multi-resilient disks and erasure coding aren't available.

Right, using Storage Spaces Direct in two or three nodes configuration is not the best idea. There are a lot of solutions in this niche like HPE or Starwind. HPE Storevirtual works with two nodes and additional witness node. Starwind works with two nodes by design. https://www.hpe.com/us/en/storage/storevirtual.html