Hi lacy11040 and welcome to the Mattress Unddrground 
decided to go the DIY route for a king-sized mattress… Hoping to get any thoughts/feedback as this will be my first attempt.
Great! Do It Yourself builds can provide the customized comfort and support, but not for the feint of heart! It can take a while to drill down to the ideal feel for you…but if you have the patience, it’s a good plan.
I’ve experienced some back and shoulder pain from our 4 year old Sleepy’s Relax mattress
Anyways, we went around to a few different mattress shops in town and both really enjoyed the Urban Mattress The Highland Mattress 1 in Firm. However, we were not really hoping to spend $2500 on a mattress (it also felt particularly steep for this mattress from what I have read), so I started trying to find something similar pre-built but ultimately decided to DIY
Not surprising the Sleepy’s Relax is no longer working out, as they disclose no infirmation at all about the type of foam, thickness or density of the comfort layer; this in many cases can indicate an excess amount of inexpensive, low density foam which can prematurely break down, sagging and causing ‘ridges’ and ‘dips’ which can cause discomfort as even alignment issues. Keep in mind the build you go with with be unique to you both, based on your Stats (height, BMI, sleeping position(s) and any underlying health concerns) - thanks for providing those - and your PPP (Posture and alignment, Pressure relief, and Personak preferences). The Urban Highland mattress, with 2 HR foam layers totalling 3" seems sound but agreed the price point is a bit high.
You can compare the Mattress Specifications You Need To Know to the Mattress Durability Guidelines to see how different materials hold up over time. For foam, 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; you may want to consider a denser foam than what you propose in your DIY build for increased durability and support.
My DIY:
- Support Layer: Caliber Edge 6" Pocket Coil (13.75 gauge) 3
- Comfort Layer: Talalay Blended 2 inch (Medium #28) 1 (bottom) + 2" Medium-Firm, Temperature Responsive Foam Topper (3 lb/ft3 , 18 IFD) 3 (top)
- Cover: SleepLikeABear All-Natural Knit Bamboo and Cotton Zipper Cover 3 (King 10")
Your build looks good so far; though for larger sleepers a lower 12-13 gauge coil may provide better support, 6" instead of 8" coils is a good choice. The bamboo cotton cover will provide breathability as well as a FR layer, and the 28 ILD Talalay latex is a good medium firmness. Do keep in mind if the top comfort layer is too soft, you may sink in too far, bottoming out on the latex layer and innerspring support, so it’s important to make sure the foam layer is of good density. there is a bit of a red flag with this layer 2" Medium-Firm, Temperature Responsive Foam Topper (3 lb/ft3 , 18 IFD) and for higher BMI’s this will wear out the fastest. But it’s good you are doing DIY - as that layer can be changed when it starts giving signs of fatigue.
A number of the Trusted Members of the site specialize in DIY components which you want want to take a look at, like APM, SleepEZ DIY Mattress DIY Natural Bedding, Latex Mattress Factory, Luma Sleep and The Mattress Factory, among others, carry components and customized sleep systems, and have a lot of experience in helping guide consumers to their ideal sleep solutions, using quality materials, full transparency in construction and components, and generous return and exchange policies. Hopefully our local DIY afficionados can also weigh in with their thoughts. Hoping you find a great sleep system that will last for many years!
~ Basilio