"Is there an option that does not involve paying the $999/year Xamarin license? (Note that, at this stage, I am not too worried about targeting multiple platforms.)"
Basically, no. VS support is only available above the "Indie" level:
https://store.xamarin.com/[
^] - so if you want to use C# to develop for Android, you gotta pay. every year.
This is why I am still not currently developing for Android: I'll have to bite the bullet, and learn Java...:sigh: