Grid changeover switching
Your approach will cause a gap in your power to the appliances ,in fact if you program the thing to have no gap the relays will blow up ,remember that the load in general is not entirely resistive .You must think of the relays as a safety component which would be essential for product approval.If you need seemless switching which good UPs do then solid state devices are needed to back the relays.Wave point switching like what you say is valid but it is unfortunately load dependant ,Not a show stopper with a micro BUT your relays will be too slow and innacurate for this to work anyway .