I don't have any experience with Cornish X but I'll offer some thoughts.
What do you mean by "improve leg quality"? More thigh and drumstick meat compared to breast meat or maybe that the legs don't break down as easily? In my opinion, the medical problems with legs breaking down are not due to any specific weakness in the body but is due to growth rate. Their skeleton's growth can't keep up with weight gain. The parent stock for the Cornish X have to live long enough to breed and lay eggs. That's mainly accomplished by restricting feed. Any genetic weakness in the skeleton that increases mortality in them would not be a good thing. I'd think about any dual purpose cross will be OK as far as skeletal strength.
As for foraging, several people on here report success in getting CX to forage. Some don't. I think that is more a matter of technique than breeding. Reading through different threads in this section you can find different techniques people use ti increase CX foraging. Some of those are how you feed or house them. One person likes to raise some dual purpose with them to show them how to forage.
I don't have any recommendations as to which breeds might increase the ratio of leg meat to breast meat. I just don't know enough. But if your goals are stronger legs and foraging, I think those are more technique than breeding. Any Dual Purpose breed would probably be OK for those goals.
Are you planning on butchering this cross or create your own line by breeding the offspring of the cross? That would influence my decision. People that do this say they have problems keeping the hens alive for more than one laying season. Even one laying season is a challenge but they do it. They don't lay tremendously well either, compared to most dual purpose breeds, maybe 150 eggs compared to over 200. If you are making this cross to eat you may be rising CX every year for breeding stock. Again, I'd think abut any dual purpose rooster would work. If you are going to try to breed offspring egg laying may be a criteria.
Do you skin or pluck? A white or buff colored bird gives you a prettier carcass if you pluck. A darker bird shows pin feathers much more and can be a pain to pluck, especially if you butcher them while going through a juvenile molt. If you skin that doesn't matter. I'd expect the first generation of this cross to be white, not an issue. There are people on here that can correct me in this if I'm wrong on this or anything else. But if you breed offspring the other colors and patterns will start to show up in the second generation. Should feather color be one of your criteria?
We all have our own different goals and criteria. We all have our personal preferences. They are often different. The people on this forum that have crossed CX are no different. Most or them are pretty good at telling you why they like the breeds they chose. Your choices should be based on your goals and preferences, not mine or anyone else's. I think you are in a good place to get some good information. Pay attention to how they manage them too. I think your chances of success depend more on how you manage them that what breed you choose.
Good luck!