Should I use DF (AFCI + GFCI) breaker for circuit sharing bathroom lights?
I would not use a dual function breaker on the lighting for several reasons.
The first being the lighting circuit usually has the fan and possibly a humidistat , timer I have had trouble with GFCI’s tripping and fans my best guess is the emf on start and turn off is the cause not always but often enough that several of the owners that had me install them had me remove them.
This is the main reason a minor reason is electronic breakers tend to have a shorter life than common breakers dual function just one more thing to fail.
As far as the switch electrical contacts these are isolated from the metal, this and the fact that it is grounded really is the protection needed if the worst happens the ground will cause the breaker to trip.
I do not see an advantage in this case and only more costs and possible failures.