For future generations looking to solve this problem:
I believe one of my modules (MMM-CTA for the record) poops out a ton of log entries. Those fill up my SD card after a few weeks.
I ended up installing a pm2 module called pm2-logrotate. https://www.npmjs.com/package/pm2-logrotate
I’m setting it to rotate my var/log file before it gets too big. Hopefully this will solve future problems.
pi@raspberrypi:~ $ pm2 install pm2-logrotate pi@raspberrypi:~ $ pm2 set pm2-logrotate:max_size 2G