You can use this tool to check what FW your clean rawnand.bin backup is on :
https://gbatemp.net/threads/firmware-version-inspector-get-fw-version-from-nand-dump.515308/
After that you'll need to get the boot0 and boot1 of this firmware. One possible way to do that is to downgrade your sysNAND to the same FW as your rawnand.bin with ChoiDUjourNX (don't forget to check the factory reset option or your switch won't boot after being downgraded), and then to backup the boot0 and boot1 with hekate. If you pair these boot0 and boot1 with your clean rawnand.bin and restore all that it should work.
Needless to say, before doing this you should first do a full backup (rawnand, boot0 and boot1) of your current 9.1 "dirty" sysNAND and put it somewhere safe on your PC, so if anything goes wrong with the downgrade you can always restore these and go back to where you are right now.
It would be easier and safer if you could do the downgrade part on an emuMMC partition and then backup boot0 and boot1 from said emuMMC, unfortunately hekate doesn't allow to backup directly from emuMMC partitions for now. It is a planned feature though, and CTCaer wrote in his last hekate/nyx update "Lastly, keep an eye for the big Nyx update" so it may very well be added quite soon and waiting for that option is also a possibility.