I would like to see the impact that setlists have upon the ratings. It is likely easier for a show to get higher ratings when song selection is strong. Initial excitement over setlist may cloud judgement on how good the playing/jams were. On the flip side, a show may not gain as much favor or excitement initially when the setlist is not as strong. Figuring out if this is the main factor would likely be tough. You would first have to figure out some kind of rankings for how popular each song is among the fanbase. To then have to go through every show and determine whether it has a setlist that would be strong on paper would probably be difficult. However, using the examples you used one could definitely argue that Gorge 3 and Forum 1 have much stronger setlists than the BGCA 2 show. Those two showed a rating decline over time and the BGCA show showed a rating increase over time.
My theory would be that setlists are the biggest contributing factor to bias in the initial ratings. Some shows with strong setlists may also just be all around great shows, in which case their rating may not change much. You see this with the Alpharetta 2018 shows for example. This theory may not be entirely possible to test without being somewhat subjective, but based on my own personal experience I think setlist can have a huge impact in the way a show is viewed upon first impression.