Heat pump controller - off the shelf
 

I was looking around on AliBaba and came across this:
Free shipping AIR 80 air source heat pump controller home heat pump PC board water heater swimming pool heat pump controller-in Heat Pump Water Heaters from Home Appliances on Aliexpress.com | Alibaba Group

A ready made heat pump controller - so I bought one to see what it was like.
Results are good - it performs well and you can configure various parameters.

The only thing I needed to do was add a relay to the reversing valve output to invert it as my machine defaults to cold and this unit defaults to heat.

At just over 40 euros it is an ideal solution for anyone who doesn't have the electrical or programming skills to build their own.

Acuario

The control only measures down to -10C so I have no idea what it will do below that. Not useful for heating where I am. Your pool stuff should be good though.

If I read it right defrost is only on at -1C to 9C. That is rather limiting.