Infill on P1S suffers from under-extrusion every time after about 5mm

Did the problem come along with a new roll of filament? That is the way to tell because in fact you may have great luck with filament arriving dry until it does not one day. You do make a point about the lower parts of the model, which makes me wonder… The S1 is 55C max which is not quite the 65C generally required for drying PETG, not a huge deal but it will take longer. So I can imagine the following scenario:

The one hour pre-heat gets 5mm worth of filament dry on the outside of the roll, just where you need it… What you need to do is dry it for 12 hours plus overnight and then print and i would venture to guess that the 5mm will increase.

I really think the drying is the issue due to the look of your gyroid infill, classic wet filament look, what you can do is when you stop the bad print, lower the bed and pinch off any excess filament with tweezers. Now, set the nozzle temp to whatever you are using (255C) and when it reaches that extrude some filament with the manual controls and quickly pinch it off with tweezers, does it now continue to come out of the hot end? It should not but if there is moisture it will be expanding the filament while it boils and you will get filament pushed out of the hot end.

Min layer time is a function of the filament profile, it is there to make sure if you have a very small layer it has time to set up before the next layer starts. It will usually result in a very slow print speed to make sure the layer takes seconds.

Above is the default for Bambu PETG Basic. I guess you are using the generic PETG so things will be slightly different. Note the Layer Time property, here is says that the layer should take at least 12 seconds.