Rebuild after RAID failure

[Alexi0]

I need some basic step-by-step help bringing my TS-453 back to life after RAID failure.
Alternative solutions to my plan that can ease the pain of rebuilding are greatly appreciated.

Current system status. RAID is running on 3 disks with one in a degraded (read-only) state (SMART is above the threshold). The fourth disk failed during RAID rebuild and the replacement one "does not belong to the group".
I thought I can "force" add it to the group, but apparently, it's impossible in this RAID state (or is it?).

I've backed up the data and am ready for radical steps.

The question is - what should I do, to retain the system configuration as much as possible? I have 4 new HDDs ready to go in (one is a part of the RAID - successfully added/replaced) and planned to be used in RAID 5 with simple volume.

Literally, I would appreciate step-by-step actions (without tiny details). I've done it a long time ago largely by trial and error and haven't touched anything for years.

If there is a tutorial - I would greatly appreciate if somebody can point me in the right direction.

[FSC830]

If it would be my NAS my steps would be:
If a config backup is possible save the config - but only, if diskcount and raidlevel should remain!
Remove the one new disk which is now part of the raid, attach it to a PC (USB dock) and remove all partitions.
Shutdown NAS, remove all old disks, insert all new disks, power on and start from scratch.

Restore data from backup!

That is IMHO the best way to bring back the NAS in operation.

Regards