Hi howdy2u.
Nice to have you back to our Mattress Forum. 
We’ve all been there with the impulse purchases. Ultimately it sounds like it has inspired you in your venture into the world of DIY, so maybe there is a silver lining there.
Thank you for sharing your sleeping positions and stats. This mattress would stand at 13" which can work well for combination sleepers. Theoretically you are looking at a “reasonable” setup. The Talatech layers M24 & M28 in soft and medium on top of the 8" Leggat & Platt coil unit from Arizona Pr make good sense for normal BMI ranges and side and back combination sleeping.
To echo Mattrebuild, I’d keep in mind that this is still theoretical as it is not possible to predict what might work best for you, because the first “rule” of mattress shopping or DIY building is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to make a specific suggestion or recommendation about which combination of materials and components would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you based on specs (either yours or a mattress), sleeping positions, health conditions, This is best left to personal testing and your own experiences or more detailed discussions with the people that sell the componentry you are considering. (See post #2 here ).
The coconut coir is used as a stabilization layer between the spring unit and the foundation which is helpful especially when the distance between slats is large. The zippered cover should be purchased last when you are satisfied with the support/comfort levels. The cover is necessary to encase all layers to keep them from shifting and can also provide a bit more protection in the long run versus oxidation/UV exposure for the latex.
Phoenix