Installed today, happy with it so far, especially the sound level, very quite compared to my central heating heatpump!!
Can't speak too much for the general case - but in the case of the Rheem unit its a soft start, so only requires only about 5 amps, single phase (notwithstanding the 20 amps mentioned on the brochure, which Rheem confirmed). This is well within the realm of a standard Shelly IOT device [which include timers if thats how you want to do it] which you can just wire onto the circuit - and if you have a switch for it, behind the switch. (and this is how I do it on mine) As I recall, the Rheem units also have a limited ability to turn themselves off during a peek time - but don't handle daylight savings and did not have the amount of flexibility I was after. I don't imagine a competent sparky would need to charge much more then the initial callout fee. The variant of Shelly I am using is the Shelly Plus1PM - which I believe has NZ certification and cost me about $60 for the hardware.
I was reading online that for units with greater draw you can use a Shelly type device to feed a larger relay, or sometimes get away with a "snubber", which I understand to be some kind of Capacitor that helps with the initial startup load.
David, did you put the shelly device behind the isolator switch? Am looking to do something similar as my docs say the power input is 2.5kw which is within the 16amps of the Shelly Plus 1PM.