Then you’ll want to know how to setup RAID with BTRFS. Something not to be tackled by the faint-of-heart and not something Cakamares Calamares is setup to deal with.
You’re best off doing a very selective and time consuming Arch install by hand if you’re wanting RAID.
I want to state for the record that I have no experience with RAID, only that I have investigated it somewhat during the course of trying to grok BTRFS. Which still tends to allude me.
If all you want are Subvols, what’s so hard about that?