Upgrade sequence is a good question. The easiest way may be to do that from UI, so 11.1-U7 next, and then to 11.2 from there.
11.2 supports both warden and iocage, that’s the time to change your jails. There is a conversion script, and ix have posts about that. Some folk had difficulty converting existing jails from warden to iocage - you may consider just re-creating your jails and copying the data they keep (config, metadata etc) into an external dataset from the original warden jail, then mounting that dataset in the iocage jail.
The YT I linked walks through that for Plex.
11.3 does away with warden, that’s the current stable release.
Since space is at a premium, I’d go with base jails rather than clone jails, they don’t grow with upgrades like clone jails do.
Snapshots only take up space if there are changes in a dataset. If you changed a MB, that’s the size of the snapshot.
On being short on space: Around 80% full ZFS performance starts to sharply drop. If you ever get to 100% full you may be at a point where you cannot delete anything any more because there is no space to write the new metadata. Can’t delete and free up space until the new metadata has been written, and there is no space to write the metadata.
Someone recently got into that state and had to copy all data off and then destroy their old pool to get back. Don’t get to 100% :).