Hi tm,
Wool and silk are both very breathable and would make good options but a natural fiber mattress pad or topper will also affect the feel and performance of your mattress (thicker will affect it more) and you would still have the “semi breathable” layer in between. You can read more about mattress pads in post #10 here.
Post #89 here talks about the different types of protectors … replacing the one you have for a more breathable type may be an option.
What kind of sheets do you have?
Phoenix