I use only ST targets so I do no have any experinces with Freescale targets.
But with higher os versions, we lost some I/O lines from the FRDM-K64, like PTC16 and PTC17.
What do you mean with that? I see these both pins are dedicated for default console (printf function).
I personally use Mbed-CE for new projects, because MbedOS development is on hold for more than year. It has different cmake build system and start up is very easy (VS code with GCC) - Home · mbed-ce/mbed-os Wiki (github.com), of course it is still MbedOS 6, but with few different things and few new targets (TeenSy 4 included) and it is more flexible for fixes especially when people want help not just pointing.
BR, Jan