Raid 1 keeps rebuilding after restart

A better solution would be to purchase a small NAS (Network Array Storage) from Synology.

https://www.synology.com/en-us/products/DS223#specs

Or a pre-built TrueNAS which has some more advanced RAID ZFS features over Synology.

Caution: RAID is not backup. Meaning you shouldn't trust RAID as your only backup. Sure if one disk dies, the other should contain your data but it's possible while swapping a failed disk that the other disk suddenly fails during the time it takes to mirror the disks. Such a catastrophic event would result in complete data loss. Or something else goes horribly wrong at the precise worst moment.

Recommend either an extra external disk that you clone as a backup to the RAID 1 pair of drives. Or perhaps an online cloud backup such as Backblaze as a secondary precaution against data loss. Or just backup your most critical data in more than one way.

If you want to use software RAID, then you should only do so when all the external drives for the RAID array are contained within a single enclosure. Even then you may still sometimes encounter the array becoming degraded. There are several reasons for this including the drives are more likely to power on, spin up and go ready at nearly the same time. Also, if you bump the USB/Thunderbolt cable, then it will take all the drives down at the same time instead of just one of the drives. There are a lot of two bay enclosures out there which can be used for a software RAID, just make sure they support accessing individual drives within the enclosure...usually will be listed as JBOD.

FYI, the way software RAID is done on a Mac is not really stable...I'm not sure if SoftRAID is any better in this regard. I know our organization sometimes utilizes the basic software RAID from Disk Utility, but other times will utilize SoftRAID instead (no idea why either one is chosen, except SoftRAID has better RAID & drive reporting with it).

@James Brickley is also correct on his points as well and I agree with him as well. FYI, the software RAID used by Synology NAS systems is much more stable than the software RAID used with macOS. Same with TrueNAS. I also know that Synology supports using their NAS as a destination for Time Machine backups.

My setup is a 27" iMac with 2 Raid-1 arrays (1 array for Time Machine 1 for Data). For the past week the Time Machine array has been rebuilding, reaches 100% and starts again. I've decided to fault the drive and add a member). As it's my timemachine, I have about 7tb of data to repair.