HI P3chorin.
Welcome to the Mattress Forum. 
It’s great to hear that you’ve found the resources on TMU useful in constructing your DIY, but I am sorry to hear you are still experiencing a little pain on your mattress. It’s not possible to diagnose back pain causes online, as this can be very complex and there are too many unknowns and individual variable involved. However, you can learn more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
Would you be willing to share your sleeping position (side/back/stomach/combo) and how many inches are in the polyester pillow top mattress topper you are using?
Out of curiosity, have you tried removing the topper and sleeping on the latex directly to see if this changes/improves/worsens your back pain?
Looking forward to your response and helping investigate what may be the cause of your back pain.
NikkiTMU