I did the DIY clean with the Supercell DIY Air Con Bunnings and it's worked quite well - the musty smell is 99% gone :) It took a few hours because I ended up taking off the heat pump case, otherwise it was difficult to get to the fins. One of the three lugs that hold the plastic cover on at that back broke off either while I was taking it off or before I started, three screws at the front hold fine, but it is ten or more years old, and it still works fine. The most difficult part was it takes a lot of hand muscle to spray all the liquid over the heat pump, so getting a sprayer that works via pressure might be a good modification, if it can be controlled easily.

I might get this DIY kit because the longer nozzle might mean I don't have to take the cover off. I will probably get a professional clean every couple of years and do the DIY clean every 6 months. When I get the pro in I'll watch how they do the ducted unit, which may be easier - take the metal lid off the box and spray the heck out of it. Only issue is it's in the roof space, but access is good.