Hi llamaduck,
Welcome back to the forum. Congratulations on your new platform bed and new mattress.
One thing I want to point out is that everything positioned between you and the floor actually matters when it comes to mattress support. The entire system is only as good as it’s weakest link.
It is not just about slat spacing even though your platform slats are 2.5 inches apart, which is slightly wider than Simmons’ recommendation of less than 2 inches. What really counts is the overall system. The mattress, the slats themselves, the center support, and any foundation or bunkie board all work together to create the support your mattress experiences. Even if your slats are thick and rigid and your foundation is very robust, from a warranty standpoint Simmons could still deny a claim if their stated spacing recommendation is not followed. Less than 2 inches is a guideline meant to protect both performance and warranty coverage, so while 2.5-inch spacing may work perfectly in practice, it does carry that potential risk and they will enforce it, read this conversation I had a while back.
Beyond spacing, slat thickness and rigidity are critical. There is a big difference between 1/2" thick slats and 3/4" hardwood slats as well as the type of wood used. These factors affect how the mattress distributes weight and how long it maintains its shape. Thinking carefully about every layer between you and the floor will help prevent premature sagging and keep your mattress performing as intended.
When it comes to foundations, I would stay away from the typical Amazon options unless you are buying something substantial. A proper bunkie, like from @mattressfoundations is a good benchmark because many Amazon boards are flimsy and will not meaningfully improve support. Beautyrest foundations are another option, but they tend to be taller, 5.5" inch or 9", and more expensive. A quick look at the BR website indicates that for a king foundation costing around $500, plus they do not provide details about what is under the cover or how thick the slats are, so you are essentially buying without full knowledge of what’s going on under the hood. By comparison, a similar product from @mattressfoundations is about $350 and may have more substantial support under the cover. and is a bit lower profile than BR low profile model.
This is the first time hearing about BR and their less than 2" stipulation, which is interesting as there are still complaints regarding their mattresses sagging in that 13-18 month stage of use. Now taking a quick peek at their website, unless you know to search for “slats” the average consumer would never see that FAQ they tucked in the foundation section not the regular FAQ’s.
The key is thinking carefully about every layer between you and the floor because each one affects mattress performance, longevity, and warranty coverage.
Hope this helps,
Maverick