I wouldn’t mix SW and HW raid, if you can, try to stick with just one or the other. By mixing them you’ll open yourself up to a whole can of worms if there’s a disk failure IMO
If this was BTRFS (or to a lesser extent, ZFS), then the solution would be to chuck the external USB enclosure into JBOD mode, and just add the disks to the array, where the storage will grow automatically in relation to the newly added disk, and you wouldn’t need to change your script…
Since you’re using mdadm, I believe you’ll need to add two new disks, but I’m not too sure if you can expand the space on the existing mountpoint without using a filesystem merging tool