I liked your write-up–it was very nice! I have a bunch of Goal Zero gear including a Nomad7 from 2 years ago and several Guide 10 and Switch 8 battery packs and I like their stuff quite a bit.
A couple of things in your analysis might be hurting you.
You had the panel indoors behind glass, but that cuts the output way down in my experience. Outside is better. I am sure there are practical considerations here, but if want better data, you probably need to measure outside.
You analyzed the USB output, not the raw output. It’s an 8-9V panel so the +5V USB port has to have electronics (probably a buck converter) to bring it down to a regulated 5V output. That voltage converter is not 100% efficient so some power is lost, but more importantly it probably has a drop out zone when the input solar power is too low.
You might want to mention how you got the data off your Spark and onto those nice graphs.
It takes a really sunny day to fully charge a Guide 10 pack with 4 AA’s for me and so generally it takes one and a half normal days to get it full. The Switch 8’s do charge in a normal day but don’t hold as much. I may get a Switch 10 eventually but I have a lot of batteries right now.
By the way, don’t let anyone with a newer Android phone borrow your Switch 8 battery for charging their phone–I loaned one out last June on a whitewater rafting trip and the battery overheated and shut down in just a few minutes!