I have taken to using a strap. I find that it puts the guitar in a very advantageous position. Of course, this means I had to install strap buttons on the guitar, which some may not want to do.
If you use a footstool that is angled, try this: When sitting back on the chair, the footstool should be angled upward in front. When sitting forward on the chair - about as far forward as you can without falling off - in a more aggressive position, angles the footstool so it is lower in front. You can simply turn it around to achieve this.
If you sit forward with the footstool angled upward, your legs will be pushing you back, so your lower back will push you forward, and you'll get pain. Sitting at the forward part of the chair with the footstool angled downward in front aligns your body so it's all working together instead of fighting itself.
Or else try a strap!