Hi!
I'll wait to hear from Corentin. You may have nailed it.
...Yes.
I repeat my englih is bad, I take time to decrypt your ideas and also codes. :-[
But I undustand almost all about yours greats proposals! 
Few details to clarify my intention:
I am not creating an animation movie but near, animating a puppet:
I have 3 servos. I would like to begin the second movement of the servo when the first did not finish his race.
Each servo has a specific "couple speed/angle".
More precisely: I am constructing a "Jack in a box". There is a switch front to the box.
When you turn-it ON, the box box opens slowly, Jack get up and his hand move slowly, turn OFF the switch and return quickly on his initial position at the same time the box is closing. (sorry if it's not very clear)
For the end it's easy (return to initial position and closing the box: using servo.write(0) function for each servo, it's instantaneous) but for begining I have to open the box slowly and a little moment after, the puppet get up and move his hand ... simultaneously".
So gbulmer's proposal semms to be appropriate for what I want.
The construction of the code with scenes interests me because I want "cut" mouvements to have acceleration, decelaration, stop ...
So I have to try with this proposition, and I will tell you if results are conclusives.
Thank you for this help!
You need however move away from using delay() (as it will block execution) and rather rely on time intervals.
I should take a look at your proposal BenF, it semms to be a more "flexible" function but a little bit hard to use for me, for the moment!
Thank you very much gentlemen, I will try all I can with your solutions and see you soon!