Hi The Dakk,
Welcome back to the community.
It is clear you’re dealing with a mix of alignment issues and potentially some mattress wear contributing to your discomfort. Given your body type and back-sleeping preference, it sounds like a firmer, more supportive mattress would be beneficial to help with the alignment and ease some of that tension in your upper back and neck. Hybrid mattresses like Helix Midnight, Signature Hybrid firm can be a great choice for that, especially with their balanced support and comfort layers. Sleepare carries these, I believe.
Since you mentioned neck pain and frequently adjusting your pillow, make sure the pillow you’re using is the right height and firmness for a back sleeper. You might want to try a contour pillow or one specifically designed for back sleepers to better support your neck. There is one called the Majestic Pillow that a few acquaintances have used and have done quite well with it. It is unique for a contour pillow as it has adjustable layers.
A mattress in the medium-firm to firm range (typically around a 6-8/10 firmness level) may be a best choice. Since you’re a back sleeper, you don’t want too much sinkage, but also want to maintain some cushioning to avoid pressure buildup, particularly around your shoulders, helix dusk, is one that sleepare is likely to carry.
When testing mattresses, try to lie on it for at least 20 - 30 minutes, and if you go with someone, have them look at your posture and alignment on the mattress to see if there is any sinking and for you to try and determine if there is any unusual pressure that you feel. Remember, your pillow is crucial. If you look at what I have written about pillows, you will understand how important they are as part of a good sleep.
Clearly, there are a handful of mattresses that would fit the description of what you need. Many from Trusted Members here at TMU. Either way if you have any questions on any of the 3z brands (helix, BB, Nolah, Birch, Bear, and Leesa) I am fairly versed in all of them.
Good luck with your mattress search.
Maverick