I think the reason you stated is not correct. No way even factory seats are designed to collapse or lay flat in a rollover. The idea is to keep your body as close to the center of the car while in a rollover as most injuries are due to the body hitting things outside the window.
Have a look at this video where this man from HMS talks about here in the quickfit pro video and the myths of the internet forums. Check about 5:22 in the video
Fixed back seats also typically do not have access to the 3pt buckle properly. This usually means your buckle goes over the bolsters but leaves a gap between your hip. This is also a dangerous situation as it is not holding you in properly even with the 3pt. There are a few seats here and there that do satisfy this requirement so do some research on that.
Another thing is, you also remove the side airbags from this car without adding anything back for the removed side protection. That is a risk you should be aware of.
With a roll bar/half cage, on the street, the cage is now semi-liable for your head injury without a helmet. BMW M4 GTS has padded roll cages from the factory to be able to sell in the USA.
I personally went with a quickfit pro + HANS device with stock seat to hold me better. It should be more than enough and also DOT legal
