You're going to have to get rid of the circuit to the well if you go with the larger feeder. Article 225 deals with outside branch circuits & feeders & is frequently overlooked. You're right, you aren't running a service, but you're only allowed one feeder or one branch circuit (generally speaking). See 225-30(b)

You'll also need 2 ground rods unless you are able to test the rod resistance & it ends up less than 25 ohms.

Tom

Few things are harder to put up with than the annoyance of a good example.