Thanks for the quick feedback!
Currently I am only using the two extruder heater channels however was planning to upgrade to a heated platform soon, therefore aim to keep that channel free for what it does best. I'm currently on ReprapFirmware 2 and would prefer to stay on this platform until v3 is made stable. My knowledge on all this should not be trusted for debugging...
Unfortunately only SERVO pin in the expansion port is PWM capable as far as I can tellfrom the wiki. I hadn't planned on installing a bed sensor though so this isn't a major. Will aim to port the chamber channel over to this down the line if need be or look at doubling up the extruder fans to free one of those channels up (the 3rd currently is part cooling).
Until then looks like it'll just have to be the bed channel