Welcome back michtx! Sorry for the delayed response.
Sorry to hear your DIY build is not working for you; As you are both higher BMI sleepers, there are a few areas in the build you describe that could be causing issues. First you have 6" of toppers, and the target foam topper is likely of insufficient density for your body profile. The soft Dunlop is in a ‘transition layer’ position and could adversely affect the structure of the mattress. In ‘normal’ mattress composition the layers go from ‘softest’ (top comfort layer) to firmest on bottom (this is not a hard and fast rule; some sleepers prefer all firm or medium firm, and in some cases a ‘reverse’ layering with a softer layer beneath as firmer one is warranted). In your current mattress the soft latex layer may be allowing it to ‘dip’ and bottom put on the coil unit below. The 2 3" soft toppers are also causing you to ‘sink‘ too far into the mattress which explains the ‘sleeping in a ditch’ and ‘sleeping on the side of a hill’ sensations you both are experiencing. You also might want to read this article on mattress layers to get a better idea of the different types and how they work together.
As far as the Beautyrest Jalisco Plush Eurotop… as you have found, it has a tendency to ‘break down’ prematurely. Beautyrest like many of the ‘big brands’ is not transparent in the densities or specifications of their components; it is also almost impossible to duplicate it or determine a mattress of equal comfort or support without this information. Also, keep in mind that insufficiently dense memory foam will break down quicker for higher BMI sleepers - for foams, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
We recently tried taking out the 3" serene foam and adding a 3" soft (19 ild) talaylay layer. It was WAY too soft that way. It was obvious that the talaylay was extremely contouring, but we sank into too much.
For your body profiles, you will ‘sink in’ with a too soft comfort layer. Again, thids can cause ‘bottoming out’ on the layer(s) below, which can cause aches, pains and even alignments issues. If possible, you may want to try a local showroom, just to test how different firmnesses of latex feel for you both. Also, Talalay rubber has a different feel than Dunlop rubber - it’s considered ‘bouncier’ (common comparison is the difference between pound cake and angel food cake).
We also tried removing the 3" serene, and just putting the 1" SOL Firm, 1" SOL soft and Cozy Pure topper over the coils. It felt very firm, which helped with the sinkage, but I tossed and turned all night and was so stiff and sore I could barely get out of the bed the next morning.
This is the issue with FIY builds: they can ultimately allow total customization to suit the sleepers using it, but it can also take a lot of time, moving layers around, frustration, and even exchanges. It’s definitely not for the ‘faint of heart’, as you are discovering.
I am considering purchasing a 2" or 3" medium dunlop layer to replace the 3" serene foam and placing as follows: 1" firm dunlop, 8" coils, 2" or 3" medium dunlop, 1" soft dunlop, Cozy Top topper. Any ideas if a 2" or 3" would get us closer to the support and softness we are looking for?
Again, much is a matter of preference, and much is a matter of proper support. No one can tell you if you will prefer one mattress over another. You want a soft comfort layer, but you will sink in too far. You need to find a ‘happy middle ground’ which is why in person testing can be beneficial, or consult one of the DIY experts which are Trusted Members of the site, like APM, SleepEZ, DIY Mattress or CozyPure who you are already familiar with.These can provide unmatched insight with their many years of experience in assisting consumers with DIY builds, and may prove to be more successful and less frustrating than ‘hit and miss’ purchasing of various layers…You say you need a ‘soft surface because of side sleeping’ but while side sleepers do need cushioning for their joints and pressure points, sinking too far can have the opposite effect.
I would consider consulting a knowledgeable DIY component manufacturer at this point so you don’t further cause many uncomfortable nights while you ‘dial in’ your correct and support levels. With a little guidance and consideration I’m sure you can ‘save’ much of your DIY build!
~ Basilio