My US senators office was responsive, but said that USCIS adjudicates joint N-400/I-751 according to the I-751 timeline, and since the I-751 isn’t past processing times there’s nothing they can do.
This directly contradicts USCIS’s own website FAQs, which clearly states that the N-400 timeline prevails in joint cases, so I’m at a loss.
I guess I have another senator and a representative that I could try.