I figured it out, I think… commenting out the Digital Ocean repositories in /etc/apt/sources.list, then updating the Ubuntu repos to point at the archive server let me apt-get upgrade and record zero updates, which in turn let me successfully update to 19.4 using do-release-upgrade. Nothing seems to have broken. I'll try going to 19.10 tonight, then switch to the next LTS when it's available.
What a faff.
