So a family member is having back issues due to sleeping on a medium-firm pocket spring mattress. They actually find it better for their back to sleep on the floor.
Is there a queen mattress, preferably foam only, that is super firm - almost as firm as couple of blankets on a thick carpeted floor?
We have the Slumberland Stonhenge queen size. Stonehenge is now replaced by Essex IIRC, of course it is pricier. Harmony is another one I use as a single bed, uspect cheaper build of Essex. All are the firmest and we're very happy with them. Got them at Snooze in one of their never-again quarterly sales.
My husband got the Zensleep one with both latex and foam and that's quite firm. Koala was way too soft for him
I would look for a 5cm firm piece of latex, that would achieve what you are looking for. Where do you live?
If it's a one sided mattress you can try turning it over and sleeping on the side with no comfort layer. Maybe add a topper of it's too firm.