I checked with Mario and he said that the problem is squarely on Beelink:
FWIW we did manage to track down one of these and dig into the exact details of what is wrong. Their BIOS isn’t configuring muxes properly, and it causes a hang in DCN.
The DMCUB firmware that is loaded to DCN hardware isn’t unique to Linux, it’s the exact same binary used by Windows as well and thus a Windows driver with the same DMCUB firmware binary will also be affected.
The proper solution is for them to fix the BIOS.
So the SER9 is broken as it comes and their primary customers, windows users, are vulnerable as well; an update could leave lots and lots of SER9 users even more broken than we Linux users are finding ourselves.
Keep an eye out for BIOS updates in the chance that Beelink makes a fix for the SER9.