Hi tinyorca, and welcome to The Mattress Underground 
Glad to have you here.
Sorry to hear you have been having trouble finding a new mattress, I’m glad the site is helping you. Keep in mind that any mattress is unique to the sleepers using it, based on their stats(height, BMI, sleeping position(s) and any underlying health conditions) and their PPP(posture and alignment, pressure relief and personal preferences).
It seems that your BMI is not very high, and assuming your puppy has no strong preferences(as long as you are happy, they will be too!) you should have a number of options. You don’t say whether your shoulder/neck and back/hip issues are due to your mattress or something else – of course if possible you should check in with a health provider to make sure that isn’t something more serious, if you have not already. You may have already taken a look at ourMattress Shopping Tutorial, as well as the Mattress Specifications You Need To Know and theMattress Specifications You need To Know to get a better idea of what you are looking at and the different types of mattresses and their attributes.
Since you mostly sleep on your back, you will want to definitely find a solution that gives you adequate support. You said due to your location in SE TN you are ordering online, but it sounds like you have already decided against mem latex, due to past experience. Have you looked into a hybrid? If you live close to Shelby or Hendersonville, you may want to try different mattresses at any retailer, just to get an idea of whether you might like specific types of mattresses.
[quote] I started researching other brands and came across Gardner and am very interested in their Organic Ergo model that is pocketed coils, cotton, and wool. Does anyone have any experience with this bed, the feel of it, etc.? I don’t live in the Boston area, so if I buy this bed, there’s no returning it which is why I’m looking for people who have slept on this bed. My parents have a Hastens that has a similar build with similar materials, and I like the feel of it, which is why I’m interested in the Gardner.
The other one I’m looking at is the Nest Bedding Spindle mattress which is pocketed coils and memory foam (which I like) where the memory comfort layer can be changed out. I’m considering this one because of the generous return policy.[/quote]
I looked at the Gardner Organic Ergo mattress, and though they are not very detailed on the construction, it appears to be chemical free and made with only organic cotton and wool. If this is comfortable for you, great! Some consumers do prefer a solid cotton mattress, and it may be a good option for you. There is a Post Here from February where a consumer discusses an allergic reaction to a Gardner Organic mattress, if this is a concern. I do need see any apparent issues you may have with the Ergo, from an outside viewpoint if that helps.
I am not quite sure of the other option you mention; whileNest Bedding is a trusted member of the site, and offers many different quality mattresses including a number of memory foam options – do you mean the Nest Sparrow ?, The Spindle mattress is a consumer direct mattress manufacturer but their mattresses are made with latex, not memory foam as far as I know.
~ Basilio