Hi, Dual Trace.
Your earlier comment about holding a six pound weight made me wonder. If you drop your LH down to your side, does your guitar neck stay in the same position as before? It sounded almost like you were holding the neck up, which should not be necessary. I have a gitano support on my CG. I like it as it is thin enough when folded that I can squeeze the cushioning a little more and fit it into the case while attached. I rotate it a little on the guitar so the support strap stays flat against my leg. With the gitano attached, just the weight of my right arm is sufficient to hold the neck in place. Perhaps the ergoplay will help with this. I found, though, that even with the gitano, I needed to either lower my seat or use a low-position footstool in order to keep the top of my left leg parallel with the floor. If the seat was too high for my left foot, the forward angle of my left leg tended to make the guitar slide towards my knee over time. Lowering the seat help counter that.
I have a relatively weak back also. A slouch position is certainly easier than maintaining a straight back. One of the things I am trying now is from when I used to ride a motorcycle. I rode standards (BMWs F650, R1100RT, K1200RS), not cruisers or sportbikes. One of the exercises to help keep from slouching was to lean slightly forward of center, keeping the weight over the feet. This became a very natural posture and I could ride for hours this way. A search for master yoda riding position on the BMW sport touring forum will provide further description. In applying this to guitar, the slight forward lean may help reduce back fatigue here also. This is a very slight lean, only a few degrees forward of vertical. But, the forward lean reduces my tendency to slouch compared with when I try to keep my back perfectly vertical. As a bonus, it provides a bit more room for the back of the guitar to vibrate. Perhaps others with more experience can comment on whether this lean may be a good thing or not. Since I work at a desk, another thing I do to strengthen the core posture muscles is to occasionally sit on a exercise ball at the desk. The tendency of the ball to move has me continually making small adjustments to my posture and lubricating the vertebrae in my back. I cannot stay this way very long yet, but it certainly provides more exercise than just leaning back in my chair.