There are no “good purposes” to be found in Endtown. There are some purposes that may be less evil than eating your fellow formerly-human citizens. That’s the most we can really hope for.
I don’t think you are correct about Endtown currently harvesting their own hydroponics — the last we heard of the hydroponics, they were still at least two in-universe years away. I don’t know how much in-universe time has passed since that conversation, but as far as I know the hydroponics haven’t been brought up as being viable yet.
The dittos don’t need Wally’s concentration. They only need to know what he wants. Remember the laundry that got clean even when he was asleep?