Time Machine and Thunderblade M1 Mac
Took me days to figure out how TimeMachine and Thunderblade work.
Time Machine will not back up with a Thunderblade configured as RAID 0 by Softraid. Any backup will stall. The only way to use Softraid with Time Machine is as RAID 4 or RAID 5, which will decrease your Thunderblade storage as 1 of the internal SSD disk will be used for parity storage. However if a disk fails, you cannot restore your RAID as a corrupt single Thunderblade SSD can't be changed. I don't understand why, as these are with a 4TB Thunderblade normal OWC chips. So you don't have the benefits of RAID 4 or 5 but you lose 1/4 th or the storage.
Solution is easy. Just use the Apple Disk Utility to configure you RAID. It will format it as Mac OS extended. If you add that disk to Time Machine, Time Machine will message preparing the disk, and change it to APFS. Softraid will show it as an MS DOS disk. Just don't touch it. The Mac does all the work.
I am not sure what you are seeing. the problem with Time Machine is it requires SoftRAID volumes to be APFS format. Then it will work.
I do not see any difference in testing, between RAID 5 and RAID 0. Time Machines just sees a "blob" called a volume and can back up to it. As long as it is in APFS format.