As mentioned in the other comment, the controller shipped with IR firmware, while I wanted IT firmware on it, so all drives shows up in the OS, instead of doing hardware RAID. I don't think this specific controller is more/less problematic than others, just that all the tooling around SAS seems to suck. At first I couldn't get sas2flash to work on Linux, so ended up trying to boot an UEFI shell with sas2flash.efi instead, which refused to work seemingly because of some motherboard/UEFI version/controller incompatibility.

So ultimately I used FreeDOS to finally be able to run sas2flash, so I could flash the IT firmware. Maybe I'm spoiled, but overall it's been a somewhat confusing journey.

And today I also started looking into getting LTO8 for long-term backups, probably will be even worse, judging by the docs I've gone through so far...