@matrix Actually got this solved, although I'm not totally sure what the issue was.
I had been using an SRV record (which has not been altered in ages) but the addition of a .well-known server:port parameter has fixed federation.
The documentation makes this seem like an either/or thing, so there may be something on my side (or theirs) that's having issues with the SRV record, although I can't figure out why.
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