Ok fixed it!!
While setting up my NixOS I also wanted to enable GRUB, however I forgot the efiSupport=true option so the default boot loader was still used. Now that I added that option, Grub is succesfully installed, and after booting with Grub everything instantly worked and I have 1080p 75Hz out of the box. Looks like the integrated boot loader just sucks.
Thank you anyway!