I sadly don't have my parents' level of knowledge (they sell matresses for a living) but there are multi-layered foam mattresses with different hardness levels. You ought to consider:
- How healthy your back is,
- if you sleep flat or on your side,
- if you are a coldboi or warmboi.
The difference between the slattered or springloaded bottom is dependent especially on your health situation. The latter is more expensive but has a much better fitment with whatever crazy position your body finds itself in and usually allows for certain regions of that base to have different "resistance" to stabilize you as you sleep.

Basically: The hardness of your mattress affects your entire body. Layering and thus mixing materials affects warmth and temperature. The bottom (frame? I only know the german word, "Lattenrost") CAN affect either your whole body or regions.

In addition, whats ontop matters too. If you have a pillow flat like a wet noodle, you arent lying stable. Look into slim but wide neck pillows - they are usually curved (cuz your noggin goes there) and also have varying degrees of how thick or soft they are. Picking a harder grade here compared to your mattress helps you to keep a good posture.

Thats all I can put out without knocking on my parents door. ^^; 

 Thank you so much for the information, I didn't know certain things 🤙