@firstdefence That ended up working, thank you!
In case anyone else finds themselves in this situation, I'll elaborate:
Actually, when I followed the steps in your second post, it didn't work the first time. It still only added a boarder to the top, and no white border was added to the bottom. Though I noticed that when I clicked "Unclip canvas", it made a few cm at the bottom of my document visible. I don't know how that bottom strip got hidden in the first place. But it was apparently included in my canvas, and for some reason was just being hidden in the display? I'm not sure exactly.
Anyways, undid all actions to start from the beginning. I selected Document > Unclip canvas, then expanded the document using the steps that I described in my original post. It then worked as expected.
I also tried your method, but adding the step Document > Unclip canvas before starting your steps. Doing it that way, your solution also worked.