My folding tire is not suitable for the mix. I couldn't find a really good one, so I have a decent one that is fine for the purpose, but not something I want to ride for ever.

If it is true rear tires wear flat, and front tires wear round, then maybe trying to even them out is not the way to go. Maybe in a perfect world they would make them with different transverse shapes and designs to start with. And maybe in practice the best you can do with that is to run a slightly wider tire in back. I think Heine now has us up to 42 mm tires as being optimal in rolling resistance, assuming you buy them from him, or someone else who executed on his memo.