I doubt that the belt contributes much to this particular issue. But it is always sensible to run through the recommended maintenance, including belt tightening, when troubleshooting.
Did you try taking over the entire A1 filament profile over to the P1 yet?
That should narrow things down a bit. Especially if you also take over any lower speed settings.
I started test #37 this morning where I copied every single value for both the filament and process settings from the A1 and while the waves are reduced, they are still very much present. I started the print with the front door closed then opened the door all the way about half-way through the print (marked with black marker on the print).
I have been cancelling the print in these tests to inspect the solid infill layer against the sparse infill. To my eyes, the solid infill lines seem to be very inconsistent - especially the outside lines overhanging the sparse infill - the lines look like they clump when on top of the sparse infill structure then stringy/non-existent in between.
That is very systematic and methodical approach. Kudos also for your perseverance! 
May I ask as to what observation prompted you to open the door?
With respect to the bob-string pattern, I usually associate this with major flow problems such as partial clogs or excessive drag in the filament feeder PTFE tubes.
Unfortunately, I will not be able to open up the 3mf until the end of next week to have a look at flow and speeds in those areas. That may provide further insights.
I opened the door b/c I have been chasing this down this issue with the assumption that it was likely related to cooling. Up until this point all of my tests have either been door open or door closed throughout the entire print, so I guess the reason I opened the door could be called “desperation” 
Last night I performed a Pressure Advance calibration in Orca Slicer, then reprinted my test file with updated Pressure Advance settings (0.042 in my case) and… it is definitely a flow issue.
In Orca Slicer I took the Bambu Lab PETG HF filament preset then changed the following:
Pictured below is the test print with the updated filament setting printed with the door closed:
Using the tuned filament profile in Orca Slicer, I then sliced and printed the actual model which started me down this path and am happy to report that it prints at least as well as the A1. There are no more bumps and the layer lines are more consistent.
I am quite happy to finally have an answer to this issue. Thank you to all who chimed in!
I so know what you mean 
Been there, done that, went back again 
And congrats on nailing the issue as well as reporting back on it That is very helpful for the future 
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