Manually offsetting the camera like done here shouldn't be needed (would break if the objects are larger or scaled in some dimension), that signals to me that the ED_view3d_camera_to_view_selected() doesn't work correctly. I checked that and indeed, grease pencil objects weren't handled by it. Created D11833: GPencil: Make camera "Frame Selected" logic work with GPencil objects.
Looks like this (without touching the preview rendering code):

I'm open to further tweaks to the rendering (e.g. the lens and camera angle), although I think what's in the screenshot looks pretty fine already. I think grease pencil objects can also have some depth, right? For that case I think keeping the slight angle that the camera has may be a good idea.