Hi maktukmak,
Welcome to the Mattress UnderGround community and joining as the affectionately called MUGster.
Your build seems fine, the choice of the 15.5TPS coils and miniquads seems like a reasonable and sensible choice.
A couple of observations. 2" of soft latex may not be enough for you. Opting for a 3" to create a more plush feel and give you another inch away from the coils.
The second thing you need to consider is the cover. A cover that is too tight will create a firmer mattress build, whereas a stretchy cover will offer the foam and coils room for a bit of expansion, creating a softer feel.
All of this is subjective. What one person describes as firm, another will feel is soft, just ask me, lol. Everything seems soft to me.
Another option is to add a transition layer between the coils and comfort layer, since your build shifts from coils to a soft foam.
Adding a transition layer above the coils and below the comfort layer could also be an effective way to adjust the firmness of your mattress. A transition layer, such as your 0.5 inch firm foam layer, can help distribute weight more evenly and provide additional cushioning between the coils and the comfort layer, which may result in a softer feel overall.
You may have to experiment with different layer configurations can help you find the perfect balance of support and comfort for your specific sleeping needs.
Another afterthought, SoL only uses dunlop latex. Certainly some will offer that a SoL soft dunlop latex of around a 20ILD should feel similar to a Talalay latex layer of 20ILD. This may be true on the surface (and with a 2" layer be insignificant) but there is a subtle difference, due to the support factor of the foam created between each process, whereby the Talalay will not be as firm feeling upon deep compression due to its more uniform “airy” density as compared to Dunlop’s more dense foam, which becomes more dense the deeper into the compression.
I am not completely positive this will have as pronounced affect as a tight cover in this situation, but certainly one to consider.
Hope this helps, I would invite the DIY experts at @Arizona_Premium and @Sleep_EZ to offer their suggestions.
Good luck with adding that .5" firm foam as a transition layer and see how that works.
Maverick
The experts at @Arizona_Premium and @Sleep_EZ may have some additional insight into this.