does cs-8000 replace cs-6800 ?
 

End my pain, please. Based on searches here, looking at convoluted Shimano compatibility charts, etc etc etc... way too much time invested... Time to replace the CS-6800 11-28. Can I use the CS-8000 11-28, or do I need to "stay back" ? And, looks like if I want to go to 30, I need a GS. Ya? Nah ? Thanks in advance.

Yes, the CS-6800 11-28 and CS-R8000 11-28 are interchangeable.
If you wanted to go to a 30 tooth cassette you can probably use your current derailleur as Shimano is rather conservative on the spread of what their derailleurs can cover... testing is the only way to be 100% sure.
If you would like anything over a 30 tooth cog, on the back, you would need the R8000 11-speed rear derailleur GS.

Thanks. It's on Di2. wouldn't mind "a bit lower" but it's not critical.

Definitely compatible cassettes. The Di2 derailleur does seem to work w/ cassettes like the 30t you're considering even a bit better than the mechanical part.