If you are having trouble with humidity, switching to a bigger tank will not help you.

I think you could move him into a 20g long at this point. The tank you listed the dimensions of seems too tall for a ball python but the floor area is good.

I would start offering food every 7 days if he is refusing ever 5 days.

You can mix substrates if you want, just don't let the substrate stay damp for too long or it will start to mold or cause scale rot.