Hi Benoit, there is always going to be a lot of set up involved for custom avatars. The easiest way to do it is to use states and triggers instead of layers.
Say, for example, you want users to select a hair style for the face. On the selection slide, you have pictures of all the hair options. When a user selects one, you use a variable to store their choice.
Instead of setting up a separate layer for each hair style, use one image of hair and add separate states for each of the hair style options.
You place the image with the states on all the screens where you want the custom avatar to appear and then use the variable trigger to change the state to the hair style selected when the timeline starts.
Hope this makes sense. Please let me know if you have any questions.