What's best method to synchronize with main AC voltage?
Have a look here: https://cdn.hackaday.io/files/1597066832861504/SimpleIsolatedZeroCrossDetector.pdf 

It is an improved version of the former EDN circuit: https://www.edn.com/design/analog/4368740/Mains-driven-zero-crossing-detector-uses-only-a-few-high-voltage-parts 

You may also read many related papers:

http://sound-au.com/appnotes/an005.htm

https://www.ijera.com/papers/Vol3_issue5/ES35834838.pdf

https://energy4all.ru/zero/zc.pdf