Closing the loop on this, attached are the photos. Here’s what I encountered;
Zooming in for more detail:
As I later found, greatly slowing the print on the first layer largely addresses the issue, resulting in:
A little stringing remains, but nothing of consequence.
In terms of root cause, it’s a combination of the height at which the bevel occurs and the slicer settings.
So, if so inclined, one could classify this as a false alarm, and I wouldn’t blame for you doing so. In my own mind it’s a bit of an edge case. My initial inclination was to print the shrinkage measurement tool using the same settings I now normally use, so as to minimize any possible differences between how I normally slice and print and what gets measured. Upon reflection, though, in this instance changing the first layer settings for this particular print maybe/probably doesn’t change the intended measurements.
On the other hand, raising the height at which the bevel occurs would likely also remedy this situation. At @Alex_vG if you’re willing to post the .step file instead of just the .stl file, I could try raising that height and report back.
And just to be clear, for the benefit of anyone who may be reading this, this problem does not occur if you slice the first and subsequent layers all at 0.2mm. It both was and remains a very good, well thought-out tool, and I highly recommend it. Indeed, I wouldn’t bother with all this if it wasn’t.
Reporting back: Comparing results I got from using normal speed at 0.2mm layer height and extra slow speed for the first 0.3mm layer height, the numbers come out quite close to one another. The shrinkage for the 0.3mm at slow speed came out at slightly less: 99.996% of the shrinkage number obtained from the 0.2mm first layer height print at full speed. i.e. likely within the realm of measurement error. 
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Finally I found time to look into it. One can see the problem already in the slicer: Not only the overhang becomes worse due to the extra height, but the elephant foot compensation makes it almost 100%. I already made the bevel only 30 degrees instead of the typical 45. But it still seems to be too much for your extra high layer.
Two ideas:
You could either remove the elephant foot compensation or instead increase nr of layers for elephant foot compensation to 2. Both might reduce the overhang. But then you are still left with only single layer webbing as you wrote already.
I don’t want to upload the STEP publicly, but if you want to stick to the 0.3mm first layer, I don’t mind to send it to you directly. Then you could adjust all the details. If you are still interested, drop me a PM.
For the benefit of anyone who may be reading this thread, using Alex_vG’s tool I’ve been able to dial in the shrinkage compensation for PETG such that the average X and Y dimensions are within 0.06% of perfect:
which is especially helpful when producing parts which ultimately need to fit together but are printed on different machines.
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