Hi Mike,
Welcome to the Mattress UnderGround. Happy to have you here.
Your approach to mattress shopping seems well thought out, and you’re on the right track with considering both hybrid and latex options.
The folks at @Sleep_EZ and @Arizona_Premium will likely chime in with some thoughtful advice.
Depending on your foundation, and it is recommended to have a nice sturdy, substantial foundation as your foundation is a key component to a longer lasting, durable sleep system. If you choose a slatted foundation, you will want the spacing to be 3" or less. This can help with reducing the possiblity of sagging and damaging the foam or coils depending on what course you take. You can choose slatted or solid wood platform (like the knickerbocker Em Brace 360 Platform.)
High-Density (HD) foam can indeed help distribute weight and prevent sagging, especially on slatted bases. It acts as a buffer between the springs and the slats, which can extend the life of the mattress and provide a more even support surface.
If you’re not finding 1" HD foam, you might consider using 2" HD foam instead. It’s commonly available and still serves the purpose of providing support and reducing pressure points on the slats. I am not sure why you might have difficulty finding a 1" HD Foam with a minimum of 1.8 - 2.0lb density or more with an ILD at a minimum in the upper 30’s (hd foam 1") for you base layer.
Since you and your spouse have different firmness preferences, you might want to consider creating a mattress with adjustable firmness or a setup that allows for customization. For example, using a split setup in the comfort or the entire latex build allows for a variety of customizable choices.
There are those who will offer that for over 250lb sleepers, all latex with the ability to customize is the way to go. Although, springs will likely provide better edge support.
Given their extensive experience dealing with a wide range of body profiles and preferences, both @Arizona_Premium and @Sleep_EZ are experts in high-quality materials and how they perform under various conditions. Their deep knowledge of latex and hybrid mattresses, coupled with their focus on customization, makes them excellent resources for ensuring you get the right support and comfort based on your unique needs.
Hopefully this will give you some food for though as you build your project.
Maverick