Hi Anti808.
Welcome to our Mattress Forum. 
You may wish to start by browsing some of the conversations we’ve had over the years with other Hawaii based mattress shoppers such as this research thread and maybe this one as well and there is this thread as well.
It’s hard to say for sure how soft/firm this mattress would feel to you as we all have different opinions and preferences when it comes to mattress feel. Basically, only you can feel what you feel on a mattress so while one person may claim it’s too soft, someone else may be saying it’s too firm, etc. I will say this - Back sleeping is the second most common position and has a slightly less curvy profile than side sleeping. The “gap” that needs to be filled in with the comfort layer for pressure relief is generally the small of the back (the lumbar area) and because the “gaps” in your profile are not quite as deep as in side sleeping, a slightly thinner top layer will generally work a little better. In back sleeping in other words a slightly shallower cradle is needed. A good starting point for a back sleeper is 2" of softer material on top of your mattress and then increasing or decreasing from there depending on other factors like weight, preferences, or other sleeping positions.
Is it possible to do any local testing to see how you feel laying on medium vs soft latex?
If not, SOL has wonderful customer service and they would gladly talk through all of your concerns with you and help you choose the most appropriate mattress for your needs from their inventory.
I hope this helps in some way!
NikkiTMU