Don't quite remember everything, so here's some notes
I have a separate pool for apps, so for Immich to use the main data pool to store images, I had to manually create all the directories that Immich needs in the pool I wanted them in, and then when installing I had to manually select each of those directories for Immich to use.
Select "Host Path (Path that already exists on the system)" for the type on each path in the "Storage Configuration" section and then you should be able to select file paths from other pools