So you buy a nice two shelf bookcase but it’s wobbly and you know it won’t hold much. I’ve recently gotten one and my solution was to put L-brackets on it. After installing about 8 of these brackets at the cojoining parts of the shelving, it is now completely stable and ready for use.
I started it on a futon too and did eventually get used to it but would not want to go back so I totally feel you.
Not sure what your budget / bed size is but IKEA have a range of different priced, decent quality pocket spring mattresses with good warranties. My friend ended up returning his after nearly 3 years (iirc) and they honoured it after inspecting and agreeing that the springs at the very edge on one side were faulty.
I strongly agree with the other comment on not taking second hand ones because of the risk of bed bugs.