Hi Hans,
I hope your luck with AliExpress will stay as long as possible 
The “aim” (moving LED from each side of the strip) is already the charging. But you are right. The time it takes for the LED to travel the stripe is different. Maybe I can set the sound so that it matches most of the time. For me it is ok when the sound is off for a few milliseconds or so.
I don´t get what you mean with making the loop always the maximum and that the charge will then take a moment before the LED is showing. Do you mean to code the effect in a way that every single aim and shoot takes the same time, which will mean that for a long travel (shoot at the end of the stripe) the LED will wait for a moment before start traveling? So the sound is already playing a few milliseconds before the LED effect starts?!
My “workstation” is in the kitchen. That means I also have to store everything when I´m done. And yes, this is frustrating 
Your solution with the piece of wood is a good idea and saves a lot of setup time.