You came to the internet for answers and while that seems like a great idea, that is actually a terrible idea which will lead you in the wrong direction. Inhale this through the nose...out the mouth.

https://www.youtube.com/watch?v=rvlA9UxGvSg

The only person who can know cost, firmness, bounce and heat characteristics on the planet is you. You need to visit 3 or so local mattress stores and determine which type of mattress suits you best. From a BIFL standpoint, the champion is a latex mattress, but...they are heavy as hell and expensive as hell.

If you order an "internet mattress", your only metric of purchase is the price. And shitty online opinions from people who keep their thermostat at 0 degrees Kelvin year round.

Visit stores. Lay on mattresses. Take notes. The sell pillows too, so you can take notes on them as the same logic applies. You are right, its all subjective, so stop searching online and take your ass in store. You are fixing to drop ~$1000 on something that should last you ~10 years, so put in the work.