Sorry for not answering sooner. I don't come here often. I am just beginning to get into working with the ESP32. I have mainly used the ESP8266 until now.
https://github.com/fabiuz7/Dimmable-Light-Arduino is the library I first tried (but couldn't get to work on the rev. 0 part). I didn't use it in the end, but it is worth looking at.
The application I have is for heating and I first thought I was going to use phase control. But I decided to go with a form of duty cycle control based on full half wave increments. I decided to use 100 as my base. For 25% power I use 25 half wave pulses and 75 pulses off before repeating the pattern. I think I would have been OK if I had used the first 25 cycles on and then 75 cycles off, but I wanted to distribute the on/off cycles more.
What I did was have ten segments in the 100 half cycles. If I wanted 10% power I would have 10000000000 repeated 10 times. If I needed 12% power the routine does 1100000000.1100000000.100000000.100000000.100000000...
The reason I am not using phase control is that the application doesn't need it (heating). The current product we sell does use phase control and you can hear the heat panels buzzing. The chopped up waveform creates a lot of harmonics and the higher harmonics are easier to hear compared to the original sine wave. Another reason I decided against phase control is that I don't have to put in an inductor for radio frequency noise suppression. All our products have a triac circuit like in your schematic but it also has a inductor in series with the load and a capacitor across the load. So by not using phase control I make a more "friendly" circuit and reduce the cost of the implementation. If I had a motor to control I would have to add the additional RF suppression parts for a commercial product. You may find that for your particular application it may not be necessary.
The one concern that I have with your schematic is the zero crossing detection. It really is half wave detection and you will have a slight skewing. I'm concerned that you might end up with each second period being different than the first. This produces a DC offset and AC motors don't really like that.
With phase control there can be some instability with short trigger pulses when operating near or at full power. Motors can start to growl. To prevent that you should either just put a steady dc on the optocoupler, or increase the pulse time so that there is a longer trigger period. This problem may or may not happen for you. It depends on the motor used.
As far as motors. Most AC induction motors are not designed for reduced speed operation. Also, when you use phase control to reduce the speed of a motor, the current drawn by the motor goes up. Reduced speed operation creates higher current through a significant portion of the speed range. The average current could be 30% more than the full speed running current. Just think about a motor at startup. It draws a higher current until it gets to the full speed. Running a motor at reduced speed makes it draw more current as it is trying to get in sync with the line frequency.
