I am replacing a fan-only unit with a fan-heater unit in my bathroom.

The switch for the existing fan is in a 1-gang box on a 15A, 14g circuit.
Next to the fan switch, in a separate 1-gang box is a dimmer for the
bathroom light which is on a *different* 15A, 14g circuit. The new
fan-heater requires a 20A circuit so I have pulled 12g Romex up into the
attic. The existing 15A circuit for the existing fan will be removed.

My plan is to use a 3-gang switch box for the dimmer, humidity sensing
switch for the fan and countdown timer for the heater. I want the dimmer to
remain on the existing 15A, 14g circuit. Moving it to the 20A circuit would
require rewiring of the walls sconces with 12g and a bunch of other work.

I know that I am allowed to have 2 different circuits in the same switch
box and i know that I should derate the dimmer, which is not a problem -
it's a 600 watt dimmer for two 60 watt bulbs.

However, I'm not sure if I am getting into a situation where I should be
derating the conductors to the dimmer, which would mean rewiring that whole
circuit. When I do I need to be concerned with derating the conductors that
are in a 3-gang switch box?

If it matters, the 3-gang box is 3.5 inches deep, the deepest I could find.
It will contain the switched hot wire for the 15A light circuit, the source
wire for the 20A circuit, a switch leg for the fan and a switch leg for the
heater.

Thanks.