Hi Erik in sac,
I’m guessing you may have already seen this but the mattress shopping tutorial includes this link to some of the better online memory foam manufacturers and retailers that I’m aware of.
If you are in a lower budget range similar to the Tuft & Needle then post #4 here includes more information about some of the better options I’m aware of for memory foam or polyfoam mattresses that are in lower budget ranges (including Tuft & Needle).
Posts #3 and #4 here also includes some options for latex or latex hybrid mattresses that are in a similar or lower budget range to the Casper mattress that may also be worth considering.
If your Dux is still in good condition and there are no sags of soft spots in any of the layers or spring components then a topper may be all you need (instead of a complete mattress) to add the additional pressure relief you need. There is more in post #2 here and the topper guidelines that it links to that may be helpful in choosing a topper if you decide to go in that direction.
Phoenix