Hi NIS2018.
Welcome to our Mattress Forum! 
Thank you for your appreciation.
I am sorry to hear that you are experiencing back pains with the SOL mattress. Issues with “pain” and the “slouching” that you describe are usually alignment (deep support) related. Sleep on Latex is using Dunlop latex in their mattresses, which is a very durable and supportive material that is not prone to sagging or deep impressions that could create the “slouching” you are mentioning when lying upon the mattress. You did not mention if you have the 7" or the 9" version, but either way this will generally be considered a very firm product and even high BMI individuals sleeping on their back would not experience “slouching”.
The misalignment that you are noticing in a product that is generally accepted as a very firm latex would point towards excessive sagging that could be caused by a foundation or support piece underneath the product that is giving way and not supportive enough. You did not mention what you have the product on, but you may wish to investigate this and place the mattress on the floor for few nights to see if it is any different. Of course keep in mind that everyone’s individual sensitivities are different and that you are the only one that can feel what you feel on the mattress.
I would not venture yet into looking for another product unless you investigate and pinpoint what may cause the issue you are experiencing with your current mattress. Usually lessons not learned have a way of repeating themselves and you may be better off to take your time and understand the nature of your issue before going in a different direction.
There are also some general guidelines as to what tends to cause back pain in post #2 here that you may find helpful.
I’m looking forward to your comments and feedback once you’ve the chance to investigate this a bit more.
Phoenix