Damaged Raid Array

I have a NAS with an NVME system drive and 2 12Tb storage drives. It is backed up to an identical system running DietPI and also a Synology NAS both with RAID disks (the backup is actually SyncThing running between all 3 machines)
This system is now reporting Disk Damage and I need to replace one of the disks.

As opposed to replacing the damaged disk, is it a simple matter to ‘convert’ it into a single disk setup?

If I do get a new disk, does anyone know roughly how long it will take to repair?

Thanks