You were right, and further research seems to elude to that RST doesn't TRIM RAID1 at all (although RAID0 it does). Such a shame...
What I did was:
1. Removed one SSD from that system and put it into another desktop
2. Ran CrystalDisk on that SSD as is in the new system, got 40 MB/s write
3. Used SanDisk SSD Dashboard to TRIM that SSD
4. Re-ran CrystalDisk and immediate got 164 MB/s write
After that, I sanitized the disk and put it back in, rebuilt the RAID array (which took only like 2 minutes), then removed the other drive, TRIM'd, sanitized and am now rebuilding it again. So the issue is related to RST especially in RAID1 where it never TRIMs.
I imagine I can do this in the future without sanitizing and just running TRIM from a separate client, but that's a pretty involved task. I hope Intel comes around to adding TRIM support for R1.