If i want to for example set quality to 2x the vanilla probabilities, how do i configure that? I see there is a global-like value at the top of settings "Default multiplier". But its default value is 0.3??? So if i set it to 0.6 it will be 2x vanilla?

Shouldn't it be 1.3 if it's a multiplier? The others are 1. Just hoping i can get off without changing every single quality individually. In fact i'd have default 1.0 that it would multiplier with vanilla values. I see in wiki they grow from 1% to 1.3%, etc. That's what x1.0 should mean imo.
https://wiki.factorio.com/Quality#Quality_modules

Edit: Still it seems that this multiplier is for quality power, not chance. So i still need to change the individuals...

To change the chance of your quality modules, you can adjust it in a few ways. Get started in the modules section of the settings menu.

The easiest option would be changing [Quality][Module] "Quality"effects from a value of "1" to "2", which will give you every quality module doubled in % chance. You can find that 4 options down from "Unlink Quality from Speed"
If you want to be more precise in how you adjust it, you can instead go back up to the individual tier % and adjust these individually for tiers 1, 2 and 3.
Or do both!? these two setting groups work well in tandem to dial in the curve your trying to make.
One more option would be doubling the "Next Probability" of each quality level individually, but that is the most work of all the methods.