Oh that’s exactly what I was looking for. I turned off mipmap and now everything is great.
I already found similar posts about mipmaps before but there it looked like not something I was looking for, but in your thread Pkeod explained it much better.
For others: the exact solution post is here - GO with sprite. Sprite quality while go.set_scale (Solved) - #2 by Pkeod
Thanks @Pkeod and @russelkgd!