Hi nick7790,
Welcome to our Mattress Forum! 
Chiming in your discussion for a moment. Sorry to hear about the lack of sleep due to lower back pains. Hopefully, it’s not a chronic medical condition but just an unsuitable mattress.
It looks you’ve been “past due” with changing your “deteriorating” OM mattress which means that your body would need a larger period to unlearn any of the compensating habits created in the process of adapting to an unsuitable sleeping surface in order to allow you getting much-needed rest. It would be helpful to know how long ago the first signs of deterioration showed up and also when you started experiencing back pains on the OM mattress. What was your sleep pattern during that time? How often did you use to wake up during the night? This can be a good indication of the length of time your body would need to adjust to a new sleeping system.
DIY can be a bit challenging and it may require some trial and error and perhaps additional expenses as you can see from lantern71’s DIY experiences.
In your comparisons, I wouldn’t focus too much on the ILD numbers alone unless the latex layers and type of latex are exactly the same and come from the same latex manufacturer (which yours are not). Answers to ILD questions may be a little bit more complex but if you’d like to dive in a little deeper you can peruse the topic titled Likelihood same Dunlop product is labeled differently (by ILD and "firmness") …
ILD is only one of several variables or “specs” that will determine how soft or firm an individual layer or a mattress “as a whole” will feel to different people (see post #4 here ). In addition to this, the ILD of different materials or different types and blends of latex also aren’t always directly comparable to each other (see post #6 here )
Looking at your initial options….I am glad you discarded them. 5" soft comfort layers on top of a 3" transition layer may be problematic for a 23 BMI “bony and lanky build” side sleeper #1. This is especially true for back or front sleepers but depending on your skeletal structure probably also for side sleepers like you. So I’d be concerned about your spinal alignment which can be easily forgotten after your experience on a hard mattress. On your side … your spine should be relatively straight (like it is when you look at someone from behind) and your body profile along the side of your body should be similar to your standing position (shoulders and hips in roughly the same relative position). You may want to revisit the article on sleeping style, preferences, and statistics
As far as your last train of thoughts for the revised option, you are moving in the right direction but I’d increase the support layer thickness by an inch or so because thicker mattresses are more adaptable and will “act” softer for most people. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights or parts of the body (also with sleeper #2 in mind).
I’d step a little back and not rush to one option or another for a little longer until you’ve got the chance to peruse some of the links above and listen to the subtle cues from your body as you try to find your way through the DIY. Hopefully, you can still experiment with the layers you already have and collect more data points. The aim is to get the construction that has the best chance of being a good match both for both comfort and support needs. If it is a bit firmer you can always fine-tune it with a thin toper but once the mattress is too soft you won’t be able to easily fix it without replacing some layers. Are the 3" firm Dunlop, 3" medium Dunlop, layers from your 7" mattress in a zippered cover so that you can do some testing with putting the Talalay topper on the bare Dunlop layers?
Phoenix