Have made this trip a few times, in Aero and others.
My view is it is a trip of two halves, on the East coast the scenery is more gentle but should be visited in order not to miss out on places such as Aberdeen and Arbroath for "smokies" and a wide range of distilleries.
Have always preferred April to avoid the midges and get some snow scenery. Huge range of places to stay, have always booked in advance according to expected mileage per day, some hotels (Loch Fyne etc) mostly in B&Bs. Do watch the speed though, the police have helicopters