Hi freespirit625.
Welcome to the Mattress Forum. 
It sounds like there are a number of factors at play here in your search for your ideal mattress.
First, I’d like to address the sagging you have mentioned experiencing on multiple mattresses. Sagging is a symptom of the breakdown of materials in the mattress and this occurs when the materials are not the right quality/durability for your sleeping specs (height, weight, sleeping positions).
I do not know the specs in the mattresses you mentioned, but when it comes to foams in a mattress…for your husband, our recommendation would be:
- Polyurethane foam: If your mattress is one-sided then I would look for 2.0 lb per cubic foot density or higher. If the mattress is two-sided then I would use a minimum foam density of 1.8 lbs per cubic foot or higher.
- Memory foam (or gel memory foam): If your mattress is one-sided then I would make sure that any memory foam is at least 5 lb per cubic foot. If the mattress is two-sided then I would use a minimum density of 4 lbs per cubic foot.
And for your BMI:
- Polyfoam: If your mattress is one-sided then I would make sure that the density of any polyfoam is at least 1.8 lb per cubic foot or higher. If the mattress is two-sided then I would use a minimum foam density of 1.5 lbs per cubic foot or higher.
- Memory foam (or gel memory foam): If your mattress is one-sided then I would make sure that any memory foam is at least 4 lb per cubic foot. If the mattress is two-sided then I would use a minimum density of 3 lbs per cubic foot.
It is very likely the materials in the mattresses you have used in the past have not met those standards.
Because of your wide weight differential and very specific comfort and support needs, my suggestion would be to explore the option of a mattress that allows for a side by side split. I would urge you to explore latex, as it is extremely durable, but also because even in softer versions it has a higher resilience and is more “supportive” than other foams, it will also help support the more recessed areas of your body, which is important for side sleepers, especially when addressing something as important as the spine. You can read more here about side by side layering in post #2.
Before making any purchase, I would recommend you read through our Mattress Durability Guidelines, our resource on finding your perfect mattress and the Mattress Shopping Tutorial.
In this day and age, especially with Covid restrictions, it is important that - if you are unable to test a mattress - that you have a detailed conversation with a knowledgeable and experienced online retailer or manufacturer that includes good information about your body type, sleeping style, general preferences and history, some general information about local mattresses you have tested and done well with, and any other specific information or circumstances that could affect your choice of a mattress.
A number of our Trusted Members offer split configurations and deliver nationally, so there is no need to leave the house! They are extremely knowledgable and compete with the best in the industry. If you present them with your sleeping specs, comfort/support needs, and all of the aforementioned, they will be able to help you choose the best fit from their selection for your needs.
I would suggest exploring options with the following…
Arizona Premium
Sleep EZ
FloBeds
Latex Mattress Factory
Nest Bedding
It’s possible I forgot to add someone so definitely do look through our directory as well.
I hope this helps you get started. If you have additional questions, please do not hesitate to ask!
NikkiTMU