Ok, I found the issue. I tried to re-wire everything and found that the connection at one of the motors was just slightly pulled out/not fully pushed in. Replugging it fixed the issue.

I did look at that but will try to reconnect everything again. The weird thing is that it works, it's not even inverted (which I know can be fixed via config file) but both buttons do the exact same action and it seems like the gantry can't/won't go up (but its not stuck). Which stepper exactly controls the Z axis? Is it the red middle or bottom one on my pictures? Initially I thought it was one of the white ones.

Yes, I can move it. I think it was too tight before but i loosened everything and then tightened it up again to make sure that nothing is too tight to prevent movement. That did help with the rattling and the movement. It can’t be the case because Z+ and Z- moves the whole gantry down. There is no difference between the presses, and doesn’t look like it would be struggling to move one way but not the other. I tried swapping A4988's around but that had no effect at all. I am not too skilled/experienced in electronics (besides building PC's) nor am I a native speaker so might not have understood the guides correctly but I think the issue is in the steppers. Can you take a look at how I have it set up/assembled and see if I have done something incorrectly please?