Yeah I would have to add two drives I think, dunno its first time I have used Raid 10 usually Raid5.
Just had 4 disks and did it because I hadn’t before 
)
\root is on 2 drives mirrored with \var on 4 as I had a glut of old drives after some updates to SSD.
It is possible to extend a RAID10 across additional PVs, but they must be added in multiples of the original RAID10
Raid5 might of been an easier choice, but actually the LVM schemes can mirror and stripe, the original just wasn’t as well supported as MD but I am presuming LVM2 has all areas covered and would be much easier.
Or just do it through NextCloud and the external storage app.
I am a bit out of the loop now but Btrfs does very much the same also and can scale to ridiculous sizes.