ZFS is fine with external disks and indirect access to hardware in my limited experience. Performance would not be as optimal as it could be but data integrity shouldn't be a problem. If I didn't need the space and this was my primary storage array, I'd probably opt for the increased reliability of 2 mirror vdevs. I've done something similar to what OP is suggesting with LVM combining multiple disks on my off-site backup though. I combined 1T+3T+4T disks into a single 8TB volume. Deliciously bastardized, not a single integrity issue, no hardware failures either over the several years it ran.