Hi dilbertzulu, and welcome to the Mattress Underground 
Sorry for the delayed response! And thanks to @TheCleanBedroom for your input! Unfortunately, your experience with the Casper is not surprising. Like the Sealy - which sounds like it held up for 15 years! - most of the ‘big brand’ retailers are pretty thin when it comes to specifications, densities, and materials used in their products; while some consumers may enjoy these big box mattresses, this makes it almost impossible to assess their suitability or durability for any sleeper…and any memory foam of insufficient density, or which was made with inexpensive ‘fillers’, the foam can break down prematurely, causing the foam to form ‘peaks’ and ‘valleys’ which often cause neck, back and muscle pains and can even cause alignment issues.
Remember, any mattress you both choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) - thanks for providing those! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences). As you and your spouse are lower BMI sleepers, and aside from what sounds like mattress-caused neck and back pains, you don’t mention any other health issues, so you have a variety of options for comfortable and supportive sleep.
As you can see comparing the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, latex is one of the most durable materials you can have in a mattress. The difference in ‘feel’ between Talalay latex rubber and Dunlop is that Talalay is considered ‘bouncier’ - like the difference between pound cake and angel food cake; some sleepers prefer one, some the other, all a matter of preference.
The latex mattresses you listed are all fine, as latex is latex as long as you find the correct firmnesses for you and your spouse. Some of those trial periods are not as great - it can take days to weeks to ‘break in’ a mattress and for your body to acclimate as well. Any good latex of proper density should not ‘soften’ unduly for sleepers of your size.
One option, once you do a little in person testing and decide whether you want Dunlop or Talalay, all-latex or hybrid, and what firmness is best, is to take a look at the Trusted Members of the site. Many of who carry different types of latex mattresses at a variety of price points…these have all been vetted for quality. transparency in materials and construction, great customer assistance and generous return and exchange policies with good trial periods. Giving them your Stats and PPP and mattress history, along with the feel you are looking for, they can help you find a perfect sleep solution. Wishing you success on your journey!
~ Basilio