At some stage (not for a few years by the looks), when I eventually upgrade my 5 HDD’s in RAIDZ2 to larger drives, can I do it in a way which allows me to have the five old HDD’s be a snapshot-in-time offsite backup?
No and yes.
No. If you replace each drive, one at a time, for larger drives, to increase the pool size, the old drives are not suitable for the existing data. Meaning, whence the ZFS pool moves on with the new, larger drive(s), the old drive becomes out dated and not any part of the pool. The data on them is useless
Yes, you can create either striped, RAID-Z1 or RAID-Z2 backup pool that is removable for off-site storage. Or use the old drives in another server for backups.
RAID-Zx is not as flexible as it could be.
ZFS Mirroring does allow breaking off a sub-Mirror of each vDev, and use that for a new pool. At which time, you can export that new pool, and take it off-site for archival purposes.