@Julien - is there a guide to setting Z tension you know of anywhere?
There’s some good info on general tensioning and pulley set screw slippage
https://docs.carbide3d.com/shapeoko-faq/shapeoko-3-how-to-check-the-pulley-set-screws/
The Z belt is tensioned by the tension screw pushing down on the tensioner bearing, there’s some good pics in the assembly guide here
https://docs.carbide3d.com/assembly/shapeoko/s3/#xz-assembly
Getting a screwdriver to the top of this screw is a bit of a pain, I recall being able to reach down between the Z fixed and moving plate when it was fully down with a long driver.
The video in this post has a good view of the hardware too
The belt should have a reasonable twang to it.