I would find an estimate for the potential change of use and advertise it as it stands with that information for anyone that would want to go purely residential. Otherwise you could pay a fee and someone else buy and want to change it back..
Definitely change agent if they're not doing a good job, if phone photos are good enough then market it yourself with your own, you're paying thousands for a service you couldn't do better yourself.. not one you could have been twice as competent at with no training or expertise whatsoever!
Look at the agencies used for similar properties in your area - here it would be CKD Galbraith, Savills, Condies, Strutt & Parker, Hayes Macfarlane, Retties, Knight Frank, Bell Ingram.. a few others if it were more farm oriented but these all do equivalent and most are nationals I think, just don't know which aren't.