Well water pressure drops after very little use time
After trying everything I could, attaching a hose to the farthest water line in the house to a hose and flushing water back out to check for blockages. Changing or cleaning all the filters. Bypassing every part of the system I could. I finally took the step of replacing the pressure tank and entire T assembly, and that fixed it. 

I have no idea what was wrong with the pressure tank though. I checked for water in the air compartment, and there was none. The pressure gauge always seemed to be working, and showed normal pressure drop with usage and increase when the switch turned on. There were no blockages in the T or pressure tank that I could find.

So I never found the actual problem, but it has to be something with the pressure tank. It looks like the answer to my issue was to just replace the pressure tank.