I have been through the DIY mattress process a couple of times, first using the pocket coil springs from the original Serta mattress with a new latex topper and most recently replacing the pocket coils with a latex core. https://forum.mattressunderground.com/t/serta-rebuild-update
Some observations:
The Serta pocket coils were still in fairly good condition when I replaced them, but the hard foam surrounding them had broken down badly. Don’t know if the S & F mattress uses a similar construction or not, but something to keep in mind.
The Serta foundations were a source of some of the sag problem which only got worse when the heavier latex foam replaced the original memory foam. The foundations were constructed with wire mesh spanning a wooden outside frame. The mesh deformed enough that it caused sags even with the rebuilt mattress.
Ken Hightower’s advice has been very reliable. His videos were also my source of inspiration. Good luck with your project.