Jim:
Before you abandon Classic, see if this helps:
In Library:
Navigate to the original Folder on your Hard Drive where those images are stored (NOT the Collection Folder) and, from the Library menu, choose Synchronise Folder.
You can also "Save Metadata to File" but, as far as I can see, that only affects the currently selected Photo so it only does one frame at a time.
If you do this your revised edits get saved back to the xmp with the original parent NEf. That means that both ACR and Lr can reach back to use the same information in the metadata.
The fact that there is a new Process 4 shouldn't make any difference because, if you re-open the file in an older version of the program, the image will just revert to adopting Process 3. The reason for introducing Version 4 was to support Range Masking and new features which may be developed in the future.
Auto-toning algorithms (if you use them) have changed too but work on all images regardless of the Process Version.
Range Masking brings far more precise editing to the Adjustment Brushes than we ever had previously.
I use them on almost every image which I process because it cuts down on the amount of masking which I will need to do in Photoshop.
Guided geometry is another addition.
(I believe that CS6 got neither that one nor the Dehaze filter?)
Pull two guides, one at each edge, and Lr/ACR will adjust verticals and horizontals to straighten selectively to the Guides.
People who are complaining about "Slow-down over time" have a different problem.
Software doesn't "go sour" like old milk!
Any perceived slow-down is inevitably caused either by Network issues or by hardware problems — especially with over-filled, non-maintained and fragmented Hard Drives.
