Took me the whole day. The new migrations require PostgreSQL version 15, and my Akkoma and Lemmy are using a shared database server of version 13. First shut down both services, then update Debian from 11 to 12, PostgreSQL from 13 to 15 and after all this, redeploy Lemmy to start the migrations. The new Lemmy queries use more RAM compared to the previous version, so the database was getting OOM and I needed to upgrade to a bigger instance.

Not fun, but everything works now and is stable.