Technically This may be possible since you only have 2tb of live data but it's risky so you will need another backup.
You need a minimum of 3 disks for a raid 5, so first wipe the backup disks, that's two. Then you should be able to break the primary raid 1, and wipe that drive, now you have 3 and your system is still running on the primary single disk (scary but functional). Now create a new raid5 with with 3 disks, then migrate your production data do it. Now you can add the 4th disk to the raid as a hot spare or expand the raid.

Again, just walking through how this is possible, not signing off on it being a good solution for primary performance or backups.

Indeed it is a good solution but risky and lengthy but doable I am thinking to leave backup raid array as is and follow your instructions to add more disks keep the OS running on 1 disk and takes it to RAID 6, does this make sense to you?

Yes go with RAID-6 or RAID10, do not use RAID-5

You should also be able to replace one of the Disk with a Bigger size and wait for it to sync, then replace the second drive and wait for it to sync, then either extend the current partitions, or create a new volume out of the unused space.

Please make sure you have viable backups before you attempt any reconfig of your storage (NB Schroedingers Backups)

Awesome thanks Gerald