I'd like to send notifications to users who don't use my site very often so that I don't lose user retention.

I suggest that you should store the fcm_token of all users in your database and using analytics check which user not came back in maybe 7 days and the send him the notification using this fcm_token.
You can also do one thing that maintain a last_visit field in users document and whenever user loads your site check if that user is logged in or not and if that user is logged in then update it's document by changing the last_visit field in users document. Use this field to to send notification to thoes users who doesn't visited your site in some time period. All this can be done in Firebase Functions.