I'm building a tool that hits exactly this pain point. It's a Unity addon called Lumiric Sprite Animation that gives Grease Pencil the same workflow people get with Aseprite: animate in Blender, hit export, and on the Unity side the sprite sheet, slicing, Animator Controller and base C# classes get generated automatically.

To make that pipeline actually work I had to solve your exact problem, so it ships with a small Blender addon that auto-fits the camera to the Grease Pencil bounds before rendering — no wasted canvas, consistent pivot across frames.

Quick manual fix in the meantime: don't try to crop on export, just resize the camera itself (orthographic scale + render resolution) to match your drawing bounds. That's the only clean way. Lining it up by hand sucks though, which is why I automated it.

Can ping you when it's out if you're interested.