I upgraded Ubuntu to eoan, 19.10, this morning, and WordPress broke. I was getting 502 Gateway errors, which is never comfortable.
What the 502 means is that nginx was not able to connect to the backend service, which is php7.3-fpm in this case.
I have nginx set up to forward to php-fpm to 127.0.0.1:9000. This is relevant.
The solution was really pretty simple: go to /etc/php/7.3/fpm/pool.d, and edit www.conf.
There will be a line with listen = /run/php/php7.3-fpm.sock ; change that line to listen = 9000 and now the socket connection is listening on port 9000 on localhost. Everything works again.
I’d still love to find a way to move off of WordPress without having to go through a massive porting process (and then a mad search for functionality, which is the bigger problem) but for right now, WordPress suffices.