Having the exact same problems with an LSI / MegaRAID SAS 9271-8i controller card too.
Windows Update is forcing me off 1909 but I'm hard stuck on 1909! Unable to upgrade in place to 2004 or 20H2. It will always reboot and rollback to 1909.
Tried the workaround of installing the latest (2017 lol) driver 6.714.18.0, uninstalling (not deleting the driver) shutdown, reinserting the card and booting up - but it always reboots.
Thinking it might have something to do with the BCD or a corrupt Windows installation, I tried a fresh install with the latest ISO from Microsoft, "Win10_20H2_v2_EnglishInternational_x64.iso". I'm unable to boot from the USB for a clean install because the controller will always trigger the storport.sys BSOD!
Contacted Broadcom and they've said,
"Interesting that someone was having the same issue with the storport.sys which 9271 does not use which leads back to MS disk.sys issue
If you are upgrading while the boot volume is on the 9271, perhaps you can install to a single drive attached to the onboard SATA port with the RAID controller out of the system.
Let Windows fully update, shut down and put the card in and see if it blue screens.
If not, you can clone the onboard SATA to the boot RAID on the controller if applicable.
What is unusual is that I recall last year, there were about 15 cases but the issue went cold until now so I would assume one of the workarounds works.
I have no feedback from customers stating what fixed the issue."
So I managed to install 20H2 without the controller installed, but when I tried to shutdown and re-install it again, the same BSOD appears after BIOS POST. Even tried to manually install the newer drivers but it's not helped.
Had to rollback to 1909 because after a week of wasted time... I need to actually use the PC. But now Windows Update keeps downloading and trying to force 20H2 on me :/