Your widow AS is virtually the same as my wall AC especially because the unit being replaced was not the exact same size so it needed some work to the walls (exterior and interior) for ~$300.
Your price is a little less than what I am paying for my current smaller 1 interior location Split but it was by far my lowest quote. I had used Thompson HVAC before but their quote had gone up a lot since I last used them for a Split (but was still far cheaper than Carini HVAC and they have done good work for me quite a few times). I got a quote from Carini HVAC based on recommendations from Bigger Pockets and Angies List and their price was outrageous.
Here is my current quote for 2 units (so divide by 2, but does not include electrical to the exterior units) and it is similar to who you used (i.e. it is a side business):
(2) Daikin 18,000 BTU Mini split Heatpumps. Includes all necessary piping and place into service. Equipment, materials and labor $5850.
It does not include electrical but was my lowest quote. So it is a little higher than what you paid especially if yours included the electrical (which is costing a fortune for these units because the units are currently on 30 Amp subpanels so I am replacing subpanels, getting a new drop from SDG&E, running new feed to the subpanels, and the feed from subpanels to the exterior units) all without any attic or raised foundation.
What size was your split? Also seeing your price is better than mine I would be interested in who you used (for next time).
The AC cost that I indicated in initial post is an average and meant to be all inclusive of all work. Some of the units were not splits but traditional AC. One of the splits was a much larger unit (I believe 36000 BTU) that required a panel swap) to accommodate a 4BR/2 Ba SFH.
For example the current quote is $2925 per unit but there will be maybe $2500 (2 new subpanels, runs from the drop to the subpanels, run from the subpanels to the exterior units) in electrical work bringing the cost for this to $4,175/unit and these are smaller size Splits with only one interior unit. Of course I am getting a better electrical solution to each unit (newer everything, more power) also but the only reason the electrical is being upgraded is to accommodate the Split HVAC.
I use laborers/handymen for most of the smaller items (demolition or maintenance more than capital expense items). I pay my labor $15/hour for less than full day work (3-6 hours)and $14/hour when the work is full day (>6 hours). My handyman gets $18 hour and is only OK but the price is right and I trust him (I used to have a great handyman but now he does REI himself). My GC is reasonable (I think he and a helper are $60/hour (for both of them total)). I currently do not have a cheap plumber but I use my handyman for most plumbing issues. I think my plumber is $81/hour when I do need to use him. So I would also be interested if you have a cheap, good plumber. I have a cheap electrician but he is real slow, not very reliable, can get overwhelmed but he is family (so I would not recommend my electrician but I use him because he is family).