Seeking Advice on DIY build.

Apologies in advance for the long post. As per the title, I’m seeking advice and a second check on what I’ve learned so far.
I’m posting in the Arizona Premium Mattress forum because I intend(ed) to purchase components from them or their companion site.

[ First; Information and Preferences: ]

[li]Mattress for one, Twin XL.[/li]

[li]Height and Weight: 5’10" 315 lbs.[/li]
→ Weight carried mostly in the stomach, some in the thighs. Losing weight slowly, about 25 lbs over the past year.

[li]Lumbar recess depth is about 3" and shoulder to chest is about 6".[/li]

[li]Primarily sleep on my back, occasionally on my side.[/li]
→ However I’m not sure if I’m only ending up on my side due to discomfort from my ~19 year old thick-pillowtop mattress.

[li]I do sleep hot, so I am concerned about breathability.[/li]
→ For this reason I also probably don’t want to be too much further “in” the mattress than needed for good support.

[li]Considering DIY for the ease of changing components either for adjustment or repair, and wanting to avoid fiberglass fire barriers.[/li]

[ Second; The initial build plan: ]

[li]After my first round of reading, I had a straightforward rough idea; 3" 28 or 32 ILD Talalay on a 6" L&P non-zoned Pocket Coil.[/li]
→ Chose the 6" non-zoned coil in case I do really sleep on my side occasionally, adding a layer won’t make the mattress too thick.
→ Would seek advice on cover options, protectors and other bedding for my heat issues after getting the layering right.

[li]But after encountering some of the information below, particularly the calculators, I’m not sure if it was a good plan.[/li]

[ Third; What (I think) I’ve learned: ]

[li]That Talalay ‘feels’ about 4 ILD softer than the same rating of Dunlop; Does this difference apply to support concerns as well?[/li]

[li]That Pocket Coils might not be appropriate for my weight; I read that the 8" Bolsa coils are roughly mid-30s ILD in firmness?[/li]
→ I did read that the 6" version would behave firmer as it’s pre-compressed further, but I didn’t see any rough ILD comparisons.

[li]That 9" thickness would either be marginal or is outright insufficient, I’ve seen some varying input on this.[/li]
→ While I understand that DIY shouldn’t be approached for saving money, I also don’t want to go overboard if it can be avoided.

[li]I have tested mattresses in person, however I’ve found I’m not picky and don’t have ‘immediate’ comfort issues with any I’ve tried.[/li]
→ The concern (and problem with my current mattress) is one of proper support throughout the night.
→ I wake with only minor aches and stiffness, but it’s enough to know there’s a problem.

[ Fourth; The Showstopper: ]

[li]I read a post that mentioned SleepEz and FloBeds having recommendation calculators on their site.[/li]
→ They suggested I need much firmer than I thought from seeing APM’s options and extrapolating from the guideline articles I read.
→ Both suggest ~44 ILD support layers and SleepEz’ also suggests a 37-40 ILD transition layer, neither of which are available from APM.

[li]I’m not sure how I should proceed from here so I’m hoping for expert advice.[/li]

I would suggest going all latex and forget the spring idea. I have many people well beyond your weight on all latex with great success. For the base I would go with Dunlop firm in the 35-39 D-85 range. Since you are a side sleeper I would go with a Talalay 3" in #32. Start there and then if you need another 2" of cush you can add it. Wait to get our cover until you have the firmness where you want it. That way we can build the cover to the exact height you end up with.

Moving away from springs was the conclusion I came to as well, but I got stopped again and decided to ask for advice when I saw how high the suggested ILD’s were on the calculators.
Since you have a great deal of experience, I can safely assume they’re just being overly conservative?

Regarding being a side sleeper, as mentioned I’m not entirely sure if I actually am a combination sleeper or if I’m a back sleeper and turn from discomfort while asleep.
For more details, I always fall asleep on my back as the most comfortable position, but I wake up on my back about 70-75% of the time and on my side about 25-30% of the time, either way my sheets being pulled around tells me I turn a lot throughout the night.
Should I stick to the same suggestion, take some time for everything to adjust then go from there if anything needs to change?

And for another question that I somehow forgot in the first post, having read up on the differing behaviors between natural vs synthetic latex with regards to compression modulus and firming vs softening with age, would you suggest either all natural or blended layers in particular, or is it not significant enough a difference?

Testing shows that blended is about 10% more durable than all natural. Sounds like you are more of a back sleeper so I would stick with my recommendation using the #32 as the comfort layer. Like I said, you can always add another softer layer if you feel it’s necessary later on.