I would:
try to create a working install that works under UEFI (because this will be easy to maintain in the future)
- use GPT
- install in UEFI-mode
- use btrfs
as filesystem - complete the install on one device
- enable timeshift or snapper (for possible rollbacks)
- add other devices to the main volume (btrfs device ad)
- convert to RAID
- do backups
(at least to another device)
You can find good Information about Btrfs in the wiki
Make Backups:
