Thanks for the quick reply! I haven't installed any other packages, or at least I can't remember doing so; in the Package Manager, I see 15 results and all of them have the status "Available", ie none of those are installed.
Using 'opkg' on the CLI, I do see a lot of packages of course, including ones that I don't think are needed for the features that I'm using. However, I don't know if I can delete any of them and if that would indeed free up space.
Doing a factory reset is not an option, I'm afraid - the router is out in the field for a research project, so I can't easily reset and reconfigure it.