I wanted to moderate the extremes mostly as well, but to also have AC for when I wanted to work in the garage on things. I'm in St. George and it's a 3 car garage, roughly 1,100sqft. I went with a 2 ton (24k BTU) system but I probably should have gone with a 3 ton. The 2 ton has a hard time overcoming the heat on demand. I plan to insulate the ceiling, which should help a lot.

The worst thing about mini-splits for what you want to do is that most are only easily controllable to a set temperature, not a wide range. I wanted to have mine keep things between 50F and 90F, but be easily set to a specific temperature. That's difficult with just the remote or the wifi app they come with. There are some thermostats that work with mini splits but they usually need line of sight since they work like a learning remote. Look at the Klima by Boldr. I ended up going with a Pioneer brand mini split because I could get an adapter to work with a regular thermostat, so now I have another Nest in the garage like in the rest of the house. If you go that route message me and I'll walk you through the wiring since their manual leaves a lot to be desired.