Correction - set_scale works for the y axis as well. I left out the second scale argument, and forgot that GML doesn't throw syntax errors for that sort of thing. Once this was fixed, the padding behaved as expected. Seems to be good for now!
Thanks! It actually ended up randomly working after a bit.
But it made me have another question : so I have this sprite of a button that's from my figma design, it has a precise size and all. I would like to create a button that uses this sprite (without nine slice), with hovering and all, but AutoUI seems to automatically resize it. Is there a way I can disable this automatic resizing for buttons?