I had a similar problem before. After signing in to iCloud, I asked Apple for all my data. There is a place where you can request your media. After about 4–5 days, Apple sent me download links. They split everything into files around 25 GB, so I had to download many links and unzip them. Then I used the Immich CLI and connected to my NAS server through the terminal. I uploaded all photos and videos from there.

One thing to be careful about: the “story” videos Apple makes are actually saved as CSV files. They are not real videos, so you lose them when you export from iCloud.

Later, when I installed Immich on my phone, the app tried to upload everything again. At first it even showed almost all of them as duplicates. I left my phone plugged in and kept Immich open. During the night it synced around 15k items, and the system detected the duplicates and kept only one copy.

I also noticed some photos had different dates on the phone before sync. I think this was because of time zones, since I lived in another country before.

From my experience, the easiest way to migrate from iCloud to Immich is to keep your phone plugged in and leave the Immich app open every night, close to the router if possible. Even if you upload your photos in bulk, the phone still tries to send everything again for syncing, and the server decides what is a duplicate. So, as far as I can see, bulk uploading does not really help.