Booting up from an external drive (I’ve been using Mojave and have only done Mojave systems) then within Disk Utility wipe both drives leave the blade SSD alone, repartition and install Big Sur onto the HDD, make sure it boots up and is working correctly.
Now you can re-join the SSD to the HDD into a Fusion Drive set following this guide [https://support.apple.com/en-us/HT207584|How to fix a split Fusion Drive]
Of course make a backup of anything first thats important.