My suggestion is create an "iron ration" that you need for a long rest and limit when you reward them, for story beats that would be like 4-6 encounters. Each long rest needs an Iron Ration and since they are limited, Long Rests become valuable. Need to bump of short rests to like 3 between a long rest.
Of course, if you are going to limit long rests, you will need to rebalance the encounters so they are not all hard / deadly.
