You're correct, Immich does support external libraries. To be more elaborate with my original comment, I meant inbuilt apps of Nextcloud which integrate well and complement the memories app. An example app would be the Face recognition one or Recognize if you fancy a different implementation. Nextcloud is after all an ecosystem so using memories gains you the other benefits of such an ecosystem. This might be overkill for some so it's upto your usecases.

Versions breaking is an issue since both mobile and server clients have to be on the same version. Compared to Nextcloud Memories this is not an issue. This was an issue when I've last used Immich so this may have changed since then.

The last time I looked, Immich can’t work with a existing file and folder structure without importing (copy) everything in his own structure (database). That’s a big no go for me.

In Memories, the file structure of your photos is preserved as-is. And you can run it alongside with other solutions that respect your folder structure.

EDIT: looks like Immich can work with external folders. But: Does it put pictures from my phone in that external folder or in its own folder?