The following may be off topic.
I recall my first design about 20 years ago to regulate mains voltage by using relatively low power triacs, as it is done here, though to drive more than one primary coil.
For a whole year, I tried adding almost all possible protection circuits and I also tried monitoring the current of the active triac so that the next triac could be fired when the previous one is totally off (to avoid shorting the transformer in the primary side).
I ended up with a rather complex board but most of all, it wasn't reliable as I expect it to be. And I had the courage to stop the project and start another one by using power triacs that replaced the mechanical relays of a conventional mains regulator (one of the eight triacs at the primary side should be on). Since I couldn't get triacs rated more than 40A. And I don't like exceeding half of the limit (here 20A), the maximum power of the regulator was
150V * 20A = 3000 W
The 150 V is the RMS lower limit of the regulated mains voltage.
This regulator is still working in my house though I had to replace the burned fuses sometimes, when a short-circuit happens or the mains is very low while many appliances are on.