I think it was literally called “config”. I will check my setup and provide the mount points I used here later today, if you back these up, it should work. Put some disposable data on it once you finished setup and then upgrade to a newer version to see if everything works. You can specify the image tag to use manually (or you install an older chart version).
I also pinned the postgres version to 14, not sure if I can recommend that but I had issues with DB upgrades in a docker installation, so I tend to be careful there.
Update: seems like the persistence section is sufficient; I have
persistence: enabled: true existingClaim: nextcloud-config-claim
at the end of my values file which references a volume claim (and volume) that I created manually upfront. The importand file is config.php. Back that thing up immediately and three times, print it if you have to. The secret in there is unrecoverable otherwise and needed for any repair actions.
I also use the postgresql sub-chart (by simply enabling postgresql as database) and provide a claim there:
postgresql: enabled: true global: postgresql: auth: username: XX password: YY database: nextcloud image: repository: postgres tag: "14" postgresqlDataDir: /bitnami/pgdata primary: persistence: enabled: true existingClaim: nextcloud-db-claim
Hope it helps!