If you can get all the images into a filesystem (on a NAS or similar server share), you can use the External Libraries feature in Immich (https://docs.immich.app/features/libraries/). This allows it to crunch through the media files via an async import job (a bit more reliable than having to directly upload via the web api).

In my setup, I exclusively use the external libraries feature, pointed at a read-only share from my NAS mounted onto my Immich server. (The external libraries are set to resynchronize to the database every few hours). This means I can manage all my media assets myself without worrying about Immich accidentally corrupting them, and if I eventually move off Immich, I just have a single folder of media files organized by date to port around.

The only downside is that I don't directly upload any media files directly to Immich, but that's okay. I have Syncopoli sync files from my phones (on a scheduled cadence) to an intermediary server which organizes and cleans exif data from media files before dropping it into its permanent home on my NAS share. No manual steps to get photos from my phone to my Immich instance!

This is a good usage pattern if you're absolutely married to the file structure you have and/or want to keep using the files where they are.

Not really applicable if what you have is a google takeout dump. Better in that case to import all the photos and let immich handle them moving forward using a tool like https://github.com/simulot/immich-go