Receptacle upstream from GFCI with an issue causing GFCI to trip?
Solved! I kept moving further upstream from the GFCI once I detected the open neutral or lower voltage neutral, and I found the culprit receptacle. A working receptacle had clearly burned a bit on one of the neutral wires. I replaced it with a new receptacle and all the downstream voltages were now steady and equal on hot and neutral. The original GFCI receptacle worked and I'm all good now and the GFCI tests and resets properly!
It felt like I walked to the breaker panel in the garage 100 times while I was troubleshooting, so I got my steps in and learned a bit more about electrical work. Thanks for all the help from those who commented.