A word or expression for "Waiting until the opportune moment"?
To "bide your time" is to wait patiently for a good opportunity to do something.
Bide your time (Cambridge Dictionary)
To bide one's time is probably the best choice.
Another one is:
To sit tight: (1) to remain in place in readiness or expectation of something, (2) to remain out of sight
Sit tight (Meriam-Webster Dictionary)
Examples:
We're planning to sit tight until the end of the game.
He told us to sit tight and wait for the captain's signal.