Rachio is supposed to have a local API coming soon. The Smart Hose hardware is great quality and I’m happy with their app in the meantime. Waiting to see how they open the integration before I commit heavily.
A stepper would be the easiest thing because if you know it takes 10 steps to fully open from fully closed, you could keep track of how many steps you have sent each direction in Home Assistant and be able to display the current status. 
If you want a knob on the wall that controls this, I think somewhere behind it you are still going to need a control system that translates wall switch inputs into stepping logic. Home Assistant would be able to do that easily. 
Stupid bastards. I hope Apple and WhatsApp and Signal all just turn off service in the EU. Let the users eat these assholes alive when their apps stop working.