I assume from the fact that the upstream shutoff is affected as well that this is garden variety galvanic corrosion. Tank is 9 years old. I'm a semi-capable DIY plumber and have some experience sweating copper.

Is it still early enough to fix? Just concerned that when I torque on that copper, I may break the nipple and I have no idea whether that's repairable.

Planning install a flex line, which I've read have insulators to prevent this sort of thing. Is there anything else I should be aware of? Should the shutoff be replaced as well? Anode too?

The globe valve should be replaced. Probably little value in replacing the anode rod at this time

Thanks! So should I bother trying to replace the corroded female connector or just leave it until it fails and replace the whole shebang?

the female adapter is not corroded. the nipple is.

if it isn't wet, leave it alone.

Next time use a brass fitting to screw on to the nipple, not copper. The steel nipple will corrode a lot less with brass than with copper.