I was also experimenting with some very old photos (50-100+ years). It seems that if the color is just too far gone for correction, you can bump up the contrast a bit then change to B&W. From there, do all the usual enhancements you would normally do, blow up the canvas larger than the size you plan to keep it at, change it to B&W again (The AI tools create tiny color artifacts), then colorize. Finally, use Giga to downscale to the size you want.
I second the hotpot tool suggested above, the process I described was actually in conjunction with that.