Luca Weiss, the Fairphone developer, said in his Open Source Summit talk this month that the Fairphone 5 will be better supported than the 6 under postmarketOS for at least 1 to 2 years. This is because of the limits on his time and the fact that the processor in the 5 got a head start on development from Qualcomm, Google, etc. With the 6 he's starting from scratch.

Mike