A relay isn’t the best way to switch at these frequencies.
Your ON time is 5mS of a 20mS cycle duration. (Asymmetric 50Hz)
Look into an SSR or triac to do the switching.
Designed exactly for that purpose.
If your project can accommodate the trailing 5mS rather than the leading part of the cycle - a triac is the perfect device, as it automatically stops conducting when the voltage drops to zero.... all you have to do is find the start of the cycle, then hold off as long as you want vefore turning ‘on’ for the remainder of that cycle.
This is how cheap dimmers operate.