A lovingly designed enclosure to enable species-appropriate husbandry. With stable, a pit for wallowing, pond and Redstone-controlled gate.

To design the enclosure some possibilities:

A barn with hay bales.

A pit for the donkeys to wallow in, made of gravel.

Different types of trees, e.g. a birch forest with tall grass or short grass with acacias.
There is also a pond which is used by the animals as a drinking trough.

And for practical riding out of the animals, a 3×3 gate made of cob.
This model is extremely rudimentary, when I get a chance I will update it.

All in all, every donkey feels comfortable here.