I know how to save a track as a track template, and a project as a project template, but I'm wondering if there's anything in between. For example, I'd like to add a "string section" as a template, which would include 5 tracks with a bus. Is there any way to do that?

When you save a track template it doesn't just have to be 1 track, it can be multiple tracks.

Like Matron said, a track template can be any combination of tracks and buses. Any selected track is saved (use SHIFT or CTRL to select more) and any used buses are automatically included. Although if you have, say, a "reverb" bus in the template, and the project you're loading into already has one, that existing one will be used.