Upload 400GB Google Photos to iCloud from external drive

I have downloaded my entire Google Photos collection (approximately 400 GB) and I want to upload it to iCloud Photos.

I have tried using iCloud for Windows, but in the latest version I cannot change the location of the iCloud Photos folder from the C: drive to another drive where my photos are stored. I don’t have enough free space on C: to temporarily transfer 400 GB just for uploading to iCloud.

What is the easiest way to upload 400 GB directly from my external hard drive to iCloud Photos?