Super thanks to you and also the others that commented this. I went and made some adjustments above in the last post I made after some further inspiration and think that I've managed to get the triangle wave to a steep enough point that it should work for my needs.As has been said, you probably don't need a perfectly shaped triangle wave (for PWM control for your purpose).
For a perfect triangle wave, the width of each peak in the waveform varies linearly with height above base.
Now consider a very imperfect 'triangle' wave, as below:
Here's a plot of the positive-going peak widths (in microseconds) for each of the heights shown in blue:
As you can see, it's not quite a linear variation of width with height, but it's not far off and should be close enough for good PWM control in many applications, such as speed control.
Thank you again!!