Regarding 3: copying to a backup server does not require the server to be in the same cluster. incus copy srv1:foo srv2: works just fine (and indeed, the instance can be called “foo” on both source and destination, which would not work within a cluster). See incus copy.
Using incus copy --refresh will give an incremental copy when supported, e.g. when the source pool and destination pool are both ZFS and there’s at least one snapshot, so only the differences are copied. It’s a bit hard to find in the documentation, unless you know the term you’re looking for is optimized volume transfer.
Combined with scheduled snapshots this makes a very nice replication/DR option, although you still have to schedule the copies yourself. For DR you can quickly bring up any container on the target node or cluster.
Regarding 4: drbd and ceph are not backups - they are replicated storage. If you delete or corrupt a file, those changes will be instantly propagated. Combined with snapshots though it’s “almost” backup. You certainly don’t want to run the “dir” driver on top of CephFS though.
There’s no native drbd support but maybe one day linstor will be added, and you might even be able to use it manually today.
Using vanilla drbd you could replicate a whole partition across to another server, but then you won’t be able to move individual containers. (Possibly for a 2-node setup you could use DRBD in dual-primary mode with LVM cluster driver; I’ve never tried it and it sounds a bit hairy. That would require an incus cluster though, and every cluster really needs a third node, even if it’s just a tiny RPi to act as a tiebreaker)
You might as well use RAID1 within a server, and if that server fails (i.e. your case 1), move the disks over to another server. Using two disks in the same server allows you to use zmirror, which is much more robust than RAID1, since it can detect and correct errors.
But really I think of RAID1/zmirror/drbd as “high availability” not “backup”. That is, your server keeps running if a disk fails, but it doesn’t protect you against data deletion, corruption, ransomware etc.