Hi avqatech,
Welcome to the Mattress UnderGround! 
Thanks for sharing your details! Let’s dive into this. There’s a lot to unpack here!
“It seems from what I’ve researched elsewhere and even read on here that hybrid mattresses are the way to go for heavier people like myself.”
Correct! Generally, for higher BMI ranges, I usually recommend spring or latex support layers. This said, I would also be quite cautious about choosing a pocket coil with your weight. Pocket coils and latex can be a great combination, but you’d need very strong coils, and I think that the consensus with most manufacturers I’ve talked with is that pocket coils that would work well for most people (e.g, 15.5 or even 14.75 gauge) may not be the best choice for higher weights.
Your BMI of 41.1 kg/m² does indeed present some challenges when it comes to mattress selection. With a higher BMI, you’ll generally need more height and firmer materials, especially in the support layers, compared to someone with an average BMI. So, I completely agree with your choice of a 12 - 14 inch mattress height as a good starting point.
With a 14" mattress … the firmness would also need to go up (than what you had for a less tick 12" mattress) on average because thicker mattresses will “act” softer for most people. If you make changes to one of the specs (such as the layer thickness of the top layer) … then you may also need to make other changes to the other layers to compensate. That is to say that there is a close interdependence between those variables.
Also, a 19 ILD for your Talalay layer would be on the soft side and less durable due to the mechanical stress this top comfort layer it’s under. Soft Talalay is certainly a good quality/value choice for normal BMI, but would not be the best choice for you. I would definitely want to know more about the specifics of the pocket coils you chose. their gauge and suitability for you, and probably I would want to make sure that the pocket coils were very strong (low-gauge steel).
This is to say that most likely the 15.5 gauge would be too soft for your needs. The choice would be between the 14.75 and 13.5 gauge quad coils. The best option for you will depend on the other elements of your mattress, particularly the comfort layers. From what I can see, I’m concerned that with the current setup, you might “sink through” the softer layers and bottom out. You would definitely sink through the Talalay soft layer, as well as through the medium Dunlop to a certain extent. This could cause you to sink too deeply into the mattress, especially around the area where you carry your weight, leading to misalignment of your spine. Additionally, if you choose a higher gauge pocket coil (softer feel), you might additionally feel a bit of a “hammocking” effect, and prevent your spine from maintaining a neutral alignment.
As side sleepers, your shoulders need enough softness to prevent pressure points but enough secondary support (coming from the comfort layers) and primary support (coming from the core layer) to ensure proper spinal alignment.
When it comes to a higher BMI, firmer materials are crucial. The general rules for mattress selection still apply: Posture and alignment, Pressure relief, and Personal preferences (PPP) are the key factors. People with higher BMI typically need thicker, firmer comfort layers and a more supportive base to keep their spine aligned, and thicker and firmer comfort layers to help relieve pressure on the hips and shoulders. Additionally, materials need to be durable, as they will wear out faster with higher body weight. Durability and quality of materials are even more important in your case.
For more guidance, I recommend reading Best mattress for plus-size people - Post 3, which covers some of the specific challenges and mattress suggestions for higher weights.
“Both my wife and I are side sleepers and fortunately I like medium to medium-firm, and she likes medium-firm to firm.”
Happy marriage indeed! It seems like you and your wife have found a common ground with the medium-firm comfort level, so it looks like you’re in good shape without needing a split king; it’ll make finding a mattress a little easier.
That said, it’s really hard to predict exactly how a specific mattress construction will feel and perform for either of you, especially with the many variables involved in personal comfort. There are just too many factors at play - your body type, sleeping positions, and personal preferences - that no amount of specs or theories can replace the value of your own careful, hands-on testing, which in your case would be the best way to ensure you’re choosing something that will work for you. As you are on a DIY mattress journey, I’d recommend talking directly with the component providers to get their expert input on how everything comes together.
Fortunately, you have a great resource in @PSS - Matan from Pocket Coil Springs. He’s a seasoned professional who has worked with many manufacturers to design mattresses that meet specific goals. He knows his pocket coil units inside and out and can provide invaluable insights into how they perform in various combinations and designs. I think your best bet would be to reach out to him directly for a conversation about your needs. No one knows his products better than him, and he’ll be able to guide you in the right direction based on your specific situation. Hopefully, he’ll see your post and chime in with some advice.
That said, I can give you a bit more food for thought, in preparation for your further discussion with Matan:
Latex has a lower hysteresis (how much energy it absorbs) and, conversely, a higher resilience (how much energy it returns) compared to foams. This means it generally provides more “bounce” and quicker recovery. However, latex can also feel firmer or softer depending on how much another layer is compressed in the mattress. For example, if a pocket coil is preloaded - meaning the 10” coil is compressed into an 8” pocket (as the quadcoil is) - it will feel firmer than a non-preloaded coil unit.
Because latex is highly resilient and has point elasticity (it contours to the specific shape of the body), it can feel both softer and firmer at the same time. How a person perceives this depends on several factors, including their weight, weight distribution, body type, sleeping position, and how much they sink into the mattress. Some people might feel it as a softer surface, while others may experience it as firmer, depending on how much they sink into it and what they are more sensitive to.
For someone with a higher weight using a softer comfort layer, they are more likely to experience the firmness of the supportive layers underneath and feel the transition between the layers more acutely. This is one of those “counterintuitive” situations where a softer comfort layer might actually make the mattress feel firmer because it compresses more under pressure, revealing the firmer layer below.
You can also reach out to @DIYMattress, which offers all latex options available for DIYers and is very experienced with how different constructions and layers work with one another. The same goes with Ken from @Arizona_Premium.
I hope I didn’t overwhelm you with too much info 
Sometimes it feels like mattress shopping is a bit like trying to solve a Rubik’s cube in the dark - lots of angles to consider, but eventually, it all comes together. If you have any other questions or if something doesn’t quite make sense, just let us know.
Happy mattress hunting - and may your back thank you soon! 
Phoenix