Update for today:
FC at 5:30 PM was 11. I added a half bottle of 10% chlorine
FC at 8:30 PM was 13
I brushed after testing, cleaned the filters and then put the older filter (the one that came with it) in. I decided to brush the bottom of the shallow end since I could kind of make out a brownisn/tan area and realized that there was dead algae there and it made a cloud, which got sucked in by the skimmer eventually. But I can almost see the brush on the bottom of the shallow end. Then I figured I may as well dirty up the filter some more and vacuumed straight into the filter, and I can just barely see the vacuum head too down there! I vacuumed the entire pool as best I could for about 20 minutes and didn't even fill up the pump skimmer with leaves this time, so I think I'm making good progress on finally getting most of the solids off the bottom. Every time I vacuum I pick up a little bit of sand/grit too.
Cleaned the older filter and put it back in for the night. Still seems to be getting dirty slower but there's still particulates floating around, you can especially see them if you turn the light on at night.
I also changed out that return pipe and no more leaks!
Will test FC in the morning and add as needed.