Hi sleepsearch98 and welcome to the Mattress Underground 
Sorry to hear of your struggle with your rebuild DIY project. As you may know, any mattress you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) - thanks for providing those! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences).
It’s not surprising that the Zinus mattress wore out prematurely. The issue with a lot of the ‘big box’ brand beds are that they lack transparency in their construction and the materials used in their components, making it difficult to assess how durable and supportive they will be for snug sleeper. Excess amounts of insufficiently dense foams can wear out causing sagging, ‘diviots’ and ‘ridges’ which can cause muscle pains and alignment issues.
A DIY project as you describe can be ultimately rewarding, as long as you are willing to put in the time considering layer firmnesses and the time it may take to ‘trial and error’ to your ideal comfort level.
As you can see by comparing the the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, latex is one of the most durable mattress materials available; however only you can tell what firmness and thickness you are both comfortable on. As a higher BMI sleeper, you might benefit from a thicker comfort layer or layers - as long as the top comfort layer is firm enough that you don’t ‘sink in’ and bottom out on the firmer layers below. A lot of this is just down to personal preference.
Hard to say how well the Zinus coils are still holding up, but if they still seem responsive, coils are not commonly a ‘weak point’ in a prematurely wearing out mattress, it’s more commonly the foam comfort layers. A medium firm Talalay topper as you describe could be the perfect solution, or you may prefer, as you say, an additional topper as you stay, firm or medium firm. This is where DIY can be frustrating to folks, as only the 2 of you can truly asssess what feels supportive and comfortable. This can take some rearranging and exchanging of layers to get that true custom feel. Unfortunately there is no ‘formula’ to determine your ideal ILD; while you need to keep your support needs front and center, a lot of this is down to the personal preferences for you both.
You may want to reach out to one of the DIY Experts among the Trusted Members of the site, like Arizona Premium, Latex Mattress Factory, DIY Mattress,or SleepEZ for some guidance on the thickness and firmness of the layer(s) you plan on using; they have a vast amount of experience in helping DIY’ers with their builds and they can help[ remove some of the frustration a DIY project can cause. If you can take your time and avoid frustration, you should certainly arrive at your ideal cust Tom DIY bed!
~ Basilio