Just wanted to give my update on this - I heard back on Dec 24th, asking for additional documents (specifically, ongoing evidence of cohabitation with my common-law partner, and refilling the sponsor questionnaire). We submitted this a day later and haven't heard back. The request for additional documents did say that if we didn't submit our documents in time, they would make a decision using the documents that they have within 30 days. I assume this means they would make the decision, either way, in that time! Fingers crossed for the next week or two.
After I got the request for documents, I noticed that my background check started (now in progress), medical is passed. At least things are moving along!