What is the best way to upload a huge photo library to iCloud?

See these articles https://support.apple.com/en-in/HT204247

https://support.apple.com/en-gb/guide/system-information/sysp4ee93ca4/mac

As per this article https://support.apple.com/en-in/HT204264

When you turn on iCloud Photos, the time it takes for your photos and videos to upload to iCloud depends on the size of your collection and your Internet speed. If you have a large collection of photos and videos, your upload might take more time than usual. You can see the status and pause the upload for one day.

I can't take my iMac to an Apple store and have them do some upload trick there to push it to the icloud faster can I?

No.

All traffic to & from iCloud is via the internet. Period.

Reality is 40,000 photos is a lot of data and iCloud does not just "FTP" it to a server. There are data integrity checks ongoing and throughout the upload. That's what prevents "starting" from the beginning whenever possible.

I’m having a similar (opposite) problem. I have 30k photos on iCloud and am having trouble downloading to my Mac. Most recently when I am trying to sync via photos app on Mac, photos would be deleted from my iCloud (I have since recovered!). Not sure how to proceed to get this huge number of photos synced from cloud into my Mac. Any help appreciated!