sounds like the fan wiring might need to be reversed.

quite often the voltage output on printer boards is the opposite way round to the wiring on the fans.

In fact I think all fans I've bought in the last few years, 12 and 24 volt, have been wired up opposite to that on the boards.

I just take the plugs out of the socket at the end of the fan wire and reverse them.

A set of these is useful to have in your workshop: https://www.amazon.co.uk/Makerstack-...190527&sr=8-15

And NO you don't want to mess with the firmware on a mighty board.
Sailfish is as good as it gets and way way better than any version of marlin I've yet tried.

But try the wire reversal first.