Hi hmmwrites,
Based on your setup, a Talalay latex topper is a great idea since it’s naturally breathable and cooler than memory foam, which should help with your heat issues.
Given that your current mattress is plush with some wear, a medium to medium-firm latex topper around 28 to 32 ILD would provide support while still offering pressure relief, especially for your hips and shoulders as a side sleeper.
A 3-inch thickness might work better than 2 inches in your case since it would do more to override the feel of your sagging mattress and give you better comfort overall. Adding wool into the mix, like in a topper that has a latex core surrounded by a wool, could add natural moisture-wicking and temperature regulation while still maintaining structure and support.
Since your husband is adaptable and heavier, that medium firmness level should work for him too, whether he stays on his back or transitions to his side.
Now for the bad news.
It’s possible if not likely, that a soft, worn mattress with body impressions could cause a topper to follow those dips, especially under a 200lb body frame.
Latex is resilient and supportive, but it’s not magic, it still conforms to the surface beneath it and above it, to some extent. That said, the right topper can still help a lot. A medium to medium-firm Talalay latex topper, especially in a 3-inch thickness, can distribute your weight more evenly and reduce the feel of those grooves. It won’t completely erase the underlying softness, but it can act as a buffer and improve support and cooling. If the impressions are deep or the mattress is sagging significantly, you might also consider placing a firm base layer, like a thin piece of high-density foam or even a bunkie board, between the mattress and topper to create a more stable surface.
All of these options will come with a price tag. A nice quality latex core and wool topper in a king can range from over $700 to $3000. One of the nicest combinations of wool and latex toppers I have seen and tried was one from Millbrook. It was compressed and tufted so tightly, that it was almost like a mattress in and of itself and was a delight to lie on if your like a firm but slightly cushion sleep surface. A king topper was around $2300 at the time. Sleepez makes a quilted version. Sleep and Beyond has a wooly latex one @TheCleanBedroom has a few options. I have a 3" organic wool topper from @ONFM, but I think you might do better with the latex and wool combination or latex and horsehair combination. The Futon Shop has a few choices too.
Ultimately, you will need to take a second look at the mattress itself if it is worth salvaging. You may also need to examine your foundation. Just to be certain that is not the cause of the “grooves” or body impressions in the mattress. Additionally, you may want to read my article on inclined sleeping as your husband may benefit with his sleep apnea and you may find it a comfortable solution as well.
It is hard to say that one thing or another will solve an issue when the mattress in question has some unknowns. My guess is that 6 year old mattress from Mattress Firm is nearing end of life regardless and I am not one who likes to chase bad money with good money.
All of the solutions mentions are valid and quality solutions, but I like to employ them during the initial stages of a mattress’s life, not towards the end. Toppers are an excellent way to add longevity, target comfort, and make a too firm mattress into a luxury sleep surface. You just have to be smart about it.
Take a step back and really evaluate, your mattress, budget and what might be coming down the road a bit, before you jump into this project.
Hope this helps,
Maverick.