@dashrandom
What I actually meant was something like this 👇
https://wiki.archlinux.org/title/Convert_a_single_drive_system_to_RAID
If mdadm isn't your cup of tea, then you can even use lvconvert with a bit of resizing of your original disk (if it allows, depending on the fs type) note that this is risky though, backups should be done. I've not really tried this.