Synch from LR Classic to LR ecosystem doesn't make sense

To address your topic points-

1] There are plugin duplicate finders for Lightroom-Classic. But leave this until a bit later until you can understand how duplicates are handled by LrC. DUPLICATE FINDER Teekesselchen: Home  and my  suggestion - do not use dupe finders that are external to LrC, they just cause more problems.

2]  Do Not create a new Catalog in Lr-Classic. Sync will only work with one catalog. If you start another new catalog everything in the cloud will download again making just more duplicates and more confusion, and you lose all the edits you have done in the ‘old’ catalog.

3] Yes you can delete everything in the Cloud AFTER LrC believes it has downloaded everything from the Cloud- it does this automatically without intervention to the folders you have chosen in the Preferences [Sync] tab..  The LrC Cloud dialog will show “Synced” under Local Activity when complete.

If you then delete an image from the All Synced Photographs special collection in LrC, it will be removed from the cloud AND from any synced collections. This will clear the space in the cloud that was occupied by that image.

*If you want to preserve existing Collections in LrC-  Turn off sync specifically on the collection(s) you want to preserve. Select all photos in the Collection and remove them (the selected images) from the All Synced Photographs special collection. This will remove them from the cloud, and clear the online space that was occupied by the image.

You can then  Re-sync the collection(s) which contain the images. This will cause a smart preview version of the image to be uploaded to the cloud. These smart previews do NOT count against your online storage space quota.

I don’t suggest having all your photos from the LrC Catalog synced up to the Cloud. Only sync Collections of selected images that will show as Albums in Lightroom - much neater to find the photos in Lightroom.

“What is the point…..”  Well Lightroom-Classic was not designed to sync when first introduced about 2006. Syncing is an after-thought so has limitations.

Basically- 

1] Everything you sync UP to Lr from Collections in LrC sends Smart Previews- quite sufficient to display and edit remotely.

2] All photos you IMPORT (ADD) to any version of Lightroom (not LrC) will occupy your Cloud storage as full-size files, AND they automatically sync DOWN to the LrC catalog.

If you are only syncing photos up to the Lr cloud by marking Collections in Lr-Classic to sync, then something is amiss.  Syncing UP from Lightroom-Classic does not use any of your cloud storage for the photo previews. (Only minimal megabytes for Cloud control)

So where does the 76GB come from?

Are you importing files to the cloud version of Lightroom?

Are you using Lightroom-mobile on a phone or ipad?

Problems with syncing can often be found by opening the browser version of Lightroom  at Lightroom.adobe.com  and check for “Sync Issues” located in the left-hand panel under ‘All Photographs’.