Hey, sharing what I learned.
For when you want your character to play some animations randomly during idling, like these.

https://github.com/j3du/Random_Idle_Animation_3D
What it does
-A Base idling animation will play at the start
-After Timer count down 6 seconds AND it finds Base idling animation is playing
-A random "gesture" animation will play instead
-When Gesture animation ends it will go back to Base idling animation and restart Timer countdown
Notes
-This demo only shows the animations (no movement or nothing else).
-It has no animation blending so animation will abruptly change (for this demo, I carefully keyframed it to look smooth).
-You can change the wait time or have more gesture animations.
-Set Base idling animation to loop and gesture animations to no loop.
You will probably need some basic understanding of these
-Export/Import a 3D character animations to Godot
-Timer node
-Signal
If anyone finds I make mistake, or has other/better method, please feel free to share here :)