You can download the manjaro-architect ISO from the main site downloads page.
I think it supports RAID.
Looking at the Prepare Installation section and I don’t think it does.
Are you using hardware of software RAID?
Software RAID can be implemented using either LVM or BTRFS, so in that case it loosely supports RAID I suppose.
Carefully read the Arch RAID wiki entry and see if you can reference this setup in the Prepare Installation section of m-a.
I’m not sure if
mdadm block partition software RAID can be mounted and installed into using m-a.