The first thing we need to do is clarify exactly what you did. This is what I think you did:
Uploaded to the cloud all the images that you had in LrC, by exporting folder by folder as a catalog, then using the migrate option in LrDeskop (LrD) to import all those images. When that had finished, you had your 30k images stored as originals in the cloud. You then decided that you also wanted to carry on using LrC, so you opened it and enabled it to sync.
OK so far? If so, the question we need an answer to is what catalog did you open in LrC and started it to sync? Was it the original catalog which still contains those 30k images, or was it a new empty catalog? Until we know that, it's difficult to advise on the way forward.
A couple of things you need to know (which you would have been well advised to learn before you started this journey):
1. When you enable sync in an LrC catalog, a copy of ALL the current (and future) content in the cloud will automatically download into that LrC catalog. There is no selective sync option FROM the cloud, everything downloads.
2. Going in the reverse direction, from LrC to the cloud, only Smart Previews of the LrC originals will upload. But that upload IS selective, i.e. nothing will sync FROM LrC TO the cloud unless/until the user enables that (usually, but not exclusively, by putting images into a sync-enabled collection in LrC).
If the sequence of events is as I descibed, and it IS the original LrC catalog that you subsequently sync-enabled, my expectation is that those 30K images would download as Virtual Copies of the original images in LrC. The reason for that is that as the originals in LrC were not previously synced, the cloud will attempt to download the 30k originals, not knowing that they already exist in the LrC catalog. However, when the cloud gives the details to LrC, LrC's duplicates detection system should determine that the new images are duplicates of existing images. Not knowing what the user's intent is, it instead simply uses the metadata from the cloud images and uses it to create Virtual Copies of the LrC originals. So you should see each unsynced original in the original LrC catalog with synced VCs alongside. The VCs will contain any edit work that you might have done in the cloud.
That's the way it should work, but I have seen reports that sometimes VCs are NOT created, instead a full copy of the images in the cloud are downloaded and added to the LrC catalog as new files. So you need to check if that is the case.
If, however, you started a new catalog, then yes the full copy of the 30k originals in the cloud would download as originals and be stored locally in accordance with the location options specified in the LrC Preferences>Lightroom Sync tab.
So, what LrC catalog did you enable for syncing?
... View more