Hi sgw, and welcome to The Mattress Underground 
welcome to our forum!
My partner and I have gone through five mattresses in the past 1.5 years and have decided it might be time to go the DIY route. We could use some advice on the layering construction.
I’m very petite, 5’1, about 95 lbs. I’m 80% side sleeper, 20% back sleeper. I like the feel of a firm mattress but because of my size and sleeping position, I have a hard time with pressure points, numb arms/hands, etc.
My partner is 5’2, about 125 lbs, mostly a stomach and back sleeper. She also likes the feel of a firm mattress and her primary issue is feeling like her hips are sinking down into the mattress too much.
Whew, sorry to hear about the trouble you are having finding a mattress…5 mattresses in 1.5 years is a lot of breaking in as well as moving mattresses in & out.
As you discovered any mattress you choose will be based on you and your partners’ Stats(height, BMI, sleeping position(s), and any underlying health conditions) and your PPP (Posture and alignment, Pressure relief, and Personal preferences). Thanks for providing your Stats, you and your partner have lower BMIs and you have a lot of options that could potentially work. I am glad to see you visited the Tutorial post and the information you need to know.
We both dislike memory foam (or anything with a sinking feel) and like to feel more on top of the mattress (with the exception of me wanting to be able to get my shoulder to sink down a bit!).
Our current bed (Saatva Classic Firm with 1" latex topper and 1" bamboo topper) is much too firm. I can’t sleep because it feels like my upper body is fighting the mattress all night, and I wake up every day with a very stiff upper back. Previous mattresses (Casper Hybrid Wave, Purple 2) have felt too soft and caused mid and low back pain. We had a Tuft & Needle Original which I liked but my parter did not. We’ve both liked latex beds we’ve tested in stores (Saatva & Room & Board models).
A DIY can be the most rewarding, as well as time intensive, options for a sleep system. It sounds like you have a good start on the project, the 1.9 lb/31 ILD poly foam is of good density and firmness for a support layer for your stated preferences. If possible, you might want to try out latex mattresses at local showrooms of a few firmness levels to see how they feel. Also keep in mind Talalay Latex is considered ‘bouncier’ than Dunlop which is more ‘settled’ (the usual comparison is between angel food cake and pound cake), so you both might want to try both types as you ‘zero in’ on firmness level for a comfort level that accommodates both your shoulders and your partners hips sinking to the preferred levels. The stretch knit bamboo cover with joma wool adds a breathable layer between you and the topper which is also good.
We could use some advice on what to use for the transitional layer between the core poly foam base and the top (soft) latex layer. If latex, what firmness might we want to look at for our respective needs? Is there something other than latex we should consider here? If you have any other overall thoughts or recommendations on the construction, we sincerely appreciate your guidance!
As you have such difference between sleeping styles you may want to consider a split/ side-by-side mattress to accommodate your individual needs and preferences. As a low range BMI and 80% side sleeper you may need a bit more comfort (maybe an inch or so of medium transition layer to borrow some support and get more comfort. Your partner however as a prone sleeper would need a firmer sleepoing surface to avoid hyperextension of the spine. I’d say get same thickness layer so that the mattress has the same height for both but a firmer ILD.
As for an additional layer, yes you can add a ‘transition’ layer of latex or another foam if desired; but you should try it without first to see how it feels on your pressure points - but only you can decide on the right firmness for you both, the one drawback of DIY is that it can take a bit of trial and error to find the perfect combo. If you go with a soft topper, a medium firm transition layer of latex or some other foam might suit you. You can read more about layers from @Phoenix.
A number of the Trusted Members of the site like DIY Natural Bedding, Latex Mattress Factory, and FloBeds,
Among others, offer DIY components and latex toppers. I think with a little more research you will have the ideal DIY bed!
~ Basilio