The most foolproof way to nail instablock timings is to use regular guard and listen to the sound. Adopt this method of learning and nothing in Sunbreak will move your shield.
Short Lance Guide: https://www.youtube.com/watch?v=MOflSJDoPQE&t=134s
Shorter Lance Guide: https://www.youtube.com/watch?v=XtETrh2q5o8
Song: Savage of the Ancient Forest: The Chase, from Monster Hunter World.