The rears on my GTS wore out before the fronts and this seems to be common. Rear discs and pads are an easy DIY job with genuine parts from ECP or AutoDoc. NB ECP supply genuine OEM discs and pads See my post here viewtopic.php?f=20&t=9613&p=193397#p193397

less than you think

yes that's right. I had them as standard fit on my 2016 GTS and when it came time to change they (Michelin Latitude N rated all seasons) were no longer available

lots of other brands available that don't have Porsche tax

where did you get these figures?
Youtube video seems to debunk your view
Up here in Scotland it's cooler and wetter (average ambient temp is between 2 and 15C) so for me winters are the best option. They are also very good off road.

no difference in braking IMHO. In any event have you ever done a full emergency stop in your Mac? - it'll come to stop in a ridiculously short distance.

I run winters all year round - much much better in wet and wear is better than summers - Michelin Alpin 5 - N rated

the rears on my GTS wore out before the fronts and this seems to be common. Rear discs and pads are an easy DIY job with genuine parts from ECP or AutoDoc. See my post here viewtopic.php?f=20&t=9613&p=193397#p193397

Traveller the caliper bolts should be torqued to 30 Nm, then further tightened with another 90o turn to provide the correct final tightness – this is as per official workshop manual instructions. If you've torqued them to 160 lb/ft (220nm) they will almost certainly be over stretched.

Like I said the bolts are single use as they stretch when you torque them and shouldn't be used again.