If you're sending them back one by one you need a guide for each.
If you send them in groups then you're not as optimal (first donkey is unladen in 3 days singly, but two donkeys are unladen in 5 days, say, meaning you need to keep feeding the first until the second is done, or equivalent via load balancing).
You could have trained donkeys that might self-navigate but that's moderately unlikely. Probably better to eat them or give them away free to the countryside.
The British were able to fly bombers from tremendously far away, with a chain re-fuel strategy where the re-fuelling tanker planes are themselves re-fuelled by other tankers which then turn back. Later iterations were optimised by thinking more carefully about who should transfer fuel, to who, and when, as in this example.
Arguably Black Buck was pointless, it was certainly not pivotal in the outcome of the war, but the actual process was fascinating.
Why? Can you not assemble them in groups like other pack animals?