Box spring is trash. Not supportive enough
Our bed has the same setup — plus more than 20 leg supports — so it’s extremely sturdy. The only downside is that it’s a pain to clean underneath.
We use a medium-firm memory foam mattress, and it’s been great. My wife likes her mattresses soft, while I prefer mine firm, so we had to meet in the middle. Our previous mattress was way too soft and eventually started sagging where I slept. I called the store, told them it had sagged beyond their warranty limit, and they sent someone out to measure it. I put a few weights on it to make the sag more obvious, and sure enough, it measured past the threshold. We ended up getting our current mattress for free.
I told my wife there was no way I was going back to a soft mattress — it fucks up my lower back. I originally wanted something very firm, but the one we have now is a good compromise.