Great success!
At least for navigation part, will see how it goes with behavior trees.
Enable “Allow Client side navigation” in project settings. (otherwise SimpleMoveToLocation fails because MainNavData is null)
Disable auto possess and replication on pawn since controller would exist only on server side anyways.
Spawn pawn, spawn AI controller and call possess on it, that way controller is not null on client side.
Pawn->SimpleMoveToLocation, off you go!