Did something similar years ago, and most opto-couplers only have a switching response time around 100kHZ at best.

High precision instrumentation just uses a stable comparator to sync the AC line Zero crossing events, and times the sharp roll-off gap over several cycles to estimate the center transition event. This allows readings to be synced in the low-noise state of the power lines cycles, and time what the readings should be when monitoring the voltage, temperature, and current across the cycles.

Additionally, most >8-digit meters internal power supply will have a tuned filter for the 60Hz/50Hz line. Getting a line filter will often give a digit of precision on older equipment, and clip-on line chokes in bulk also help.

https://www.digikey.com/en/products/detail/eaton-tripp-lite/...

One final tip, anything beyond 12bits precision or 60MHz wide RF bandwidth for SDR will almost certainly have an export restriction. Wideband devices also have RF front end saturation issues, and usually require narrow band-pass filters to see the hydrogen line etc. Best of luck =3