I recommend to not have the OS on an HDD.
Best is to have both MSFS and OS on NVME or SSD.
Also the virtual memory should absolutely reside on a fast drive.
If you are on a budget you can get a cheapo 128gb SATA SSD for 20$ and have OS on SATA SSD and MSFS on NVME.
FYI I have OS/MSFS on a budget Adata 500GB PCIE3 NVME with fairly good performance thanks to ZFS caching up to 2GB.
Loading times:
MSFS takes a good minute to go pass the check update screen, I dont think it is purely disk related.
During load time, OSD shows the disk operations are very fast, but the decryption time does not depend on disk speed and takes its toll.
Before SU5 I saw 1mn30 loading times with 1 logical core at 100 per cent and the rest lower.
After SU5 MSFS starts in about 2mn20 with or without addons and lower individual core usage.
Makes sense if decrypting is the main factor.
Loading a flight on the French Riviera with TBM930 improvement mod and custom livery is around 35s in SU5.
It was faster before SU5, but I did not time it then.
EDIT: Oops: I’ve just seen the new setup. Congrats!