Murphy's oil soap and then Mink oil applied to leather after.
This was what we did eons ago on horse tack and it still works best I think.
I would be iffy about making a 'leather' soap for someone if you've not ever made it and 'tried and true' used it.
I have made similar stuff and it cleans leather pretty well.
(Note: I make turpentine soaps also to get rid of grease and dirt a bit better and have never had it go into flames when mixing with the final concoction. I add about 5 percent to the finished hot processed soap in place of fragrance oil and it still has never flamed on me.)