I will just throw this out there for what it is worth. When I first opened my pool I was in a similar situation as you. I did not know anything about pool maintenance and had a swamp on my hands. Similarly, once I started to reach the end of the SLAM process I started losing water. In my situation, though, it did not stop at the skimmer and almost all of the water in the shallow end was gone before I stopped the leaking. Come to find out it was the way the pool was plumbed. I had moved one of the valves at the equipment pad which was allowing water to go out of the pool into another pipe which, to this day, I still have no idea where it goes. I just know now not to turn the valve to that pipe
Since your leaking is stopping at the skimmer that may not be the case, but I wanted to at least mention it. If you have moved any valves or changed anything at the equipment pad it may be worth looking at those again. You may have already eliminated this as a potential cause, but just in case

- great job by the way!!