I did a similar conversion on a Serta last summer. Thread is here: https://forum.mattressunderground.com/t/serta-rebuild
Ken Hightower of Arizona Premium Mattress was very helpful on the project. He also has several mattress rebuild videos on his website if you haven’t seen them.
We have 3" of medium Talalay latex over the Serta pocket coil innersprings. My wife is very happy with the combination and wouldn’t change anything. I find the combination a bit too firm sleeping on my left side, although when on my right side it’s fine. Probably more arthritis pain on the left side is the culprit. Good luck with your rebuild! Vic