Is it possible to migrate LR Classic catalog to Cloud w/o uploading all pictures ?. The catalogues itself are already around 64 GB of storage (including preview). If all raw pictures will be uploaded at same time when migrating catalogues there will be an upload of 2.5 TB..
No, migration means storing everything in the cloud. If you do not want that, then stick to Lightroom Classic. You can sync selected images to the cloud from Lightroom Classic, so they show and can be edited on your mobile devices. Edits will sync down too. These images are smart previews, and do not count against your cloud space.