I built an 2,000 square foot addition to my house. About 900 sq. ft. of that is 17' high ceilings. Last year while I was working on finishing interior I bought an LG inverter 22kBTU window AC and kept it cool at 75F all summer long. I was really blown away by the "window shaker."
R38 fiberglass ceilings and R19 fiberglass+R5 XPS walls. High ceilings need ceiling fans to keep cool up there if you need it.
This Spring I installed a 24kBTU for just the 17' ceiling portion, as the remainder was conditioned by house central unit. The LG is going in my garage, which has a natural gas heater already.
As far as energy costs in my case, the inverter Window AC and minisplit have been the same so far.
If you ONLY need cooling, I would jump on the LG inverters. My garage doesn't have any windows and has a generous 30" overhangs, so not worried about waterproofing it as a thru-wall system.
You need to compare inverter Window AC vs inverter minisplit to make it apples to apples. Of course any inverter mini is going to be more efficient to a single-speed window AC. If they were to make an inverter Window AC WITH heat pump, that's basically a "packaged" inverter mini split heat pump.
Just purchase, unpack and drop in any available window, plug into outlet and flip to "ON," LOL.