A little breaking out of the maths for the Schmitt Trigger allowed me to move the thresholds from +/- voltages to all positive voltages.  And then I needed to shift up the integrator reference as well, by adding R7 and R8.

Now we're cooking!  Rail to rail square wave output, integrated triangle wave, all on a single supply.

Next stop threshold output from the triangle wave, gives us a fixed PWM.