Here is some research for you. No Affiliate links in my links.
Some information for research.
Fix your sagging mattress
https://www.youtube.com/watch?v=5xihmJhS-as
https://www.youtube.com/watch?v=Md-JlSi-n0Y&t=6s
Video of 50+ year old latex mattress
Go-to https://www.sleeplikethedead.com/latex-toppers.html
That website is not trying to sell you anything.
https://www.diynaturalbedding.com/the-banana-test/
https://www.foambymail.com/LTX_T/dunlop-latex-foam-topper.html
https://www.amazon.com/s?k=latex+topper&ref=nb_sb_noss_2
https://www.sleeplikethedead.com/mattress-reviews-latex.html
https://diymattress.net/products/caliber-edge-6-pocket-coil-by-leggett-and-platt
https://diymattress.net/products/latex-toppers-all-natural-dunlop?variant=34758771404
https://www.foambymail.com/LTX_T/dunlop-latex-foam-topper.html