Is there a way to set some value over the maximum value ? For exemple the loot quality to 8 instead of the max : 5 ? Thanks.

There is not. That is why it says max is 5, because that is max.

which makes no sense considering you still get all the grades EXCEPT ascendant at 5. i'm still getting primitives out of reds, REDS -.-

can always customise your loot crates. not sure if that would help with the quality in the drops but heres the link to a guide i used to help me on mine. https://survivetheark.com/index.php?/forums/topic/65930-tutorial-modify-loot-crate-contents-v242/&