In theory it should be as easy as copy and pasting, but I don't know if that wouldn't fail because of a timeout. Though, I have managed to wget a Google photos backup directly into JuiceFS and extract + process it there, just took like a day of time.
Mounting your Immich dirs there is all it should take. I would be careful with database directories/sqlite, they seem to bug out when mounted in JuiceFS
I think rclone has somewhat bad latency, at least from prior experience (albeit with Google Drive). JuiceFS seems to cache locally and can run my Immich instance, etc. With pretty good performance
True, but S3 offers you extremely high availability and security for a quite fair price, and not everyone wants to immediately self host on their own hardware.