[Dominik]

I also would try to activate the clutch fork manually to get a feel of the needed travel.
My memory tells me it must be about 2" - which depends on the ratio of the fork, of course.

That 90 deg fitting at the slave can be a bear to bleed. Those kinks lead to cavitation (bubbles), if you don't bleed very slowly. How did you bleed? By pumping the pedal?

I would make a new line, without that union and a 45 deg bend on either end, or at least a 90 deg bend - not a kink.

What diameter is your feed line from the reservoir? Fluid needs to run freely when you bleed.
5/16" ID is good.