Hi Krco and welcome to the Mattress Underground
Interesting project! A DIY build can be really rewarding as you can customize each layer for maximum comfort and support - but it can be a frustrating project as well, as it’s not really uncommon to rearrange, exchange/replace and return layers and spend a good bit of time considering factors and options to get a completed sleep solution
As you probably know, any mattress you finally consider a ‘finished’ DIY 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).
A Saatva having a ‘dip’ in the comfort layers after 6 years is unfortunately not uncommon. In your current configuration, however, as a higher BMI sleeper, you may ‘sink’ too far depending on the firmness of the SOL topper and ‘bottom out’ on the pocket coil layer below. The bottom coil unit is for extra support, and you may or may not want or need this; if possible you could put the 4” coil unit you have ‘salvaged’ on the floor with the SOL topper just to see how it feels by itself to get an idea of whether or not you want or need that bottom coil unit (as your body profile and sleep profile changes and adapts over time, you possibly would not be as comfortable on your mattress if you could revert it to its ‘new’ state now).
Much of the rest is really a matter of preference. Keep in mind Talalay and Dunlop have different ‘feels’ with Talalay being considered ‘bouncier’ than Dunlop (think of the difference between pound cake and angel food cake). As per above, you don’t want a latex layer that allows you to sink too deeply - which effects may not be noticeable immediately - as this can cause aches, pains and even alignment issues over the long term. An HD foam ‘base layer’ is common but not strictly necessary…depending on where you are, what median temperature you keep your bedroom at, and your preference, ‘closing off’ the bottom can create a ‘warmer’ sleep system, but you sacrifice some breathability. remember it’s your mattress and you and only you can tell what you prefer!
You might want to consider a thinner top comfort layer if you want to keep the ‘soft’ feel, of an inch or two so you don’t sink, after which the medium firm ‘transition’ layer; a layer of wool would create a breathable layer and is a natural flame retardant as well; Trusted Member of the site @CozyPure, for example has a 1” Tufted Wool Mattress Pad which might be of interest.
Foam ‘rails’ do provide edge support for coil units, but are not necessary - are you an ‘edge sleeper’? Do you need a firm edge to sit on the side of your bed? Again, it comes down to your preference but it’s an extra added expense and search you may want to consider before moving ahead…likewise the knit vs. quilted cover. If it’s at all possible in your area, you could try out some various mattress types with different types of firmnesses, covers, and types/firmness of latex just to get a general idea of what feels best and make a ‘short list’ of those…this could save a lot of time and effort some sleepers run into with DIY builds on ‘drilling down’ to their ideal build.
It sounds like you are on your way to a great DIY bed…just be patient and take time to consider what your next steps should be, and with a bit of patience, you will certainly be rewarded with an ideal sleep solution!