I just replaced my 20yo King mattress last year.
It is just a good quality memory foam from a national outlet. I sleep great on these, and will not spend multiple thousands on a high tech bed. Though mine was over a $K.
You go to a store in decently clean clothing, take off your shoes, and try them out (one by one).
I got a medium firmness, I could never take hard mattresses.
Memory foam holds heat (bad in summer, but never an issue in winter, fall or spring for me in VA), and mine was a version of foam designed to minimize memory foam heat more than any other in their inventory. Though I will say, with memory foam, I never have much bedding on top of me, year round (this is the other key to memory foam heat buildup). I am a human furnace and hate a hot bed (which was a long lasting marital issue, before I fired her).
Also with memory foam, the softer it is, the more you sink, the hotter the (foam) bed gets. So if you get foam, understand that the harder it is, the cooler it will be. Thus my medium hard-soft new one (5 or 6 of 10). So don't get too soft, if heat is an issue for you.
I told them my (two single) king box frame(s) were perfectly good, and I only wanted to pay for a mattress. They told me I paid 100% of the price for the mattress alone, and I got the box frames free, so I took the new box frames, and they carted the old frames and mattress away.
Decades ago, I decided I liked a lower bed, so I took the steel frame and put it my storage (no fancy waste-of-money bed furniture for me; where you bang your hands and feet on headboards and footboards). You lose the ability to store things under the bed, but I like a lower bed, esp when I get up to pee at night. Plus, you no longer have to worry about dust bunnies under a bed for cleanup. And I think you get better support from a box spring on the floor than on a frame only supported on the edges (and better mattress longevity too). If you get rid of the frame, a bit taller box spring will be no issue at all.
What I didn't like, but discovered was now standard, was that these new mattresses can be turned but not flipped. I routinely turned and flipped my old mattress and that's why it lasted 20 years. But now top and bottom is not interchangeable.
(Flipping a king mattress alone was always a challenge in my small bedroom. It was more of a wrestling match than a household chore, and sometimes it threw me and I lost and had to start over.
)
When asked why I stayed with a king after I became single, the answer was that I already had a linen closet full of good king bedding, and no queen or double bedding at all. I'm cheap. And the king was not much more expensive anyway. Now on the occasion it does heat up, I just move over 20 inches and it's cool again.
