Attach a wave to the moments it genuinely belongs to: someone arriving, someone leaving, the end of a video call.
Say the word and do the wave together, every time, in the same order — 'bye bye' with the hand moving.
Do not make them wave. Model it and wait. When the gesture eventually appears it will arrive attached to the right moment, which is the whole point.
Gestures arrive before words and carry the same communicative work. A wave tied to a real departure is learned as meaning something, whereas a wave drilled on request is learned as a trick performed for adults.