Not familiar with the nextcloud side of things, but I just pulled all my photos from Google photos and imported them all to Immich. I’d imagine if you just have a folder full of images, it’d work the same way. During the Immich setup, you can designate an import folder. Point that import folder at your photos folder that you want to bring in. Once you have Immich up and running, you can use the terminal and run an import from the command line on that import folder. You’ll have to make an API key for the CLI to use, but you can make that in the settings. Immich doesn’t currently support mass importing from inside the UI, so this is the only way I’ve found to do it. The import ran fast for me though, went though 125gb of photos and videos in about 5 mins.