The upgrade failed because the PostgreSQL upgrade required 14GB of free disk space and the droplet didn’t have quite that much.
Why it didn’t work after I expanded the disk space, might be that as suggested by @Jagster above I did a ./launcher cleanup that was inappropriate for a two-container installation.
At some point I’d like to switch to a single-container installation since that seems less problematic for someone who doesn’t really know the Discourse architecture. I would also prefer not to self-host but the site has a lot of photos and Discourse hosting jumped from $100 to $200 a month when we ran out of disk space. The site only brings in around $30-40 a month from Google Ads.