I read that PTFE starts to deteriorate past 260 °C. Does that mean heating to 250 °C is no problem at all, or will that destroy the PTFE material over time to?
Degradation starts at 260 °C and shifts towards full blown decomposition towards 350 °C. 250 °C is technically fine, but you should keep in mind that you've got little to no wiggle room for error at that temperature. Your thermistor and board may not be accurate enough to guarantee you'll never overshoot that temperature, and the way 3D printers often handle temperature adjustment exacerbates that risk. You can print at 250 °C, just be aware you've got basically no margin for error.