Hello eed1010.
Welcome to our Mattress Forum. 
Wow! Looks like you’ve tried quite the range of beds. How long did you test these beds for? Was it in-store, or did you have them home for some number of days before returning?
Okay, so so far:
- Costco
- No memory foam
- Preferably innerspring or similar to a traditional innerspring
- Looking to avoid back pain
- Side sleepers
Side sleeping is the most common position, and because it has a “curvier” profile, side sleepers need layers they can sink into for pressure relief. Without the right support, you’re likely to feel pressure on your hips and shoulders, which could lead to soreness or numbness. A good comfort layer for side sleepers generally falls between 2-4” to create a deeper cradle and distribute weight more evenly, helping to fill in gaps and reduce pressure points for a better night’s rest.
As you’ve found, the firmer mattresses are creating these aforementioned pain points.
Well, no bed is going to be a cure for back pain unless a bed is the reason for the back pain. You can read more about the reasons for back pain on a mattress in this handy post. You may also find this forum topic interesting and/or relevant.
How do you feel about this current mattress? Is it feeling too firm, too soft?
NikkiTMU