I’m sorry to say Adam that I’ve spent a lot of time thinking and experimenting with this and I’ve come up with nothing better.
My version works differently to yours, so I guess it’s inevitable that you’ll get different characteristic behaviour. It seems that, if you rely on a sync pulse for the slave oscillator, you have to take into account of what happens
before the first sync pulse arrives when you have different phase shifts adjusted.
I even decided to make my own version from the ground up, using sync, but I hit the same issue every time: the
first part-cycle was incorrect at various combinations of settings.
This may not help, but I think my last version will sound acceptable, because there is immediate consistency and the FM waveform looks reasonably symmetrical. Also, when you get a discontinuity (step) in the wave, it creates high harmonics independent of the point in the cycle where it happens. I proved this to myself using my Wavemaker 4 ages ago.
I would suggest you audition it against yours, for a sustained note, and judge the results accordingly. If mine is not fit for purpose then it would be down to someone more capable than me to figure out a better method. I did try my best!
Cheers
Spogg