I highly doubt f35 will use supersonic speed for anything else than repositioning/travel. In combat theaters i doubt it speed will go higher than Mach 0.8.

And anyway, if you want CAS, to stop an army, A-10 will probably be a dozen time better suited than any multirole, and especially the f35 with its ridiculously low availability rate, or even better in that particular case, an AC-130 (that is probably able to direct a drone fleet in its latest revision, but that was speculation last time i checked)

(the A-10 is the best modern plane in my opinion, i really like the F15, f18 and Rafale (those curves!), because i really like the idea of aircraft carrier but that plane is the best.)

Supersonic speeds would certainly be used for beyond visual range (BVR) engagements. Higher speeds mean you and your missiles have more energy and maneuverability. Since the F-35 needs to use it's afterburners to reach supersonic speeds, it likely wouldn't use the extra fuel just for transport.

The default for an F-35 is to be at something like M=0.9 and altitudes of 20-25kft.