I only replaced the under tank mat. The yellow piece behind my seats was dry and in perfect condition (and not smelly). I guess it depends from where on your tank the leaks were coming.
Also it is at least possible that even though it smells very bad in the back seat it is due to the vents into the trunk (there are 2) behind the back seat more than any of the physical material behind the seat itself.
Taking the tank out is a bit of a pain for sure. I would do everything you think you will want to while it’s out the first time so as to not have to take it out (hopefully) ever again.
I even sanded and re-sprayed the shelf that the tank sits on while mine was out.