Funnily enough I use nextcloud to upload my photos and then immich with the external dir pointed to my nextcloud directory to actually view them. Mostly came about because I installed nextcloud first. But it means I get to keep both immich and still have a regular file structure.
Same here, I tried Nextcloud Memories first which has some of the same functionality but ended up being too slow in nearly all aspects. Immich does well in this regard so I use it just like you do using its external library feature.
You may be interested in the change I submitted to Immich which makes it possible to directly log in to Immich using the Nextcloud OIDC app. Add Immich as an 'external site' and use the autoLaunch parameter [1] to have the site open in NC as if it were a 'native' app without needing to log in first.
