The Zeph. 3:8 text is a parallel to Joel 3:1-2; this is The Final Gathering at The Valley of Jehoshaphat that we see at the end of Revelation 19 where the returning Christ Jesus PERSONALLY assassinates all of finally unrepentant humanity down to the very last soul.
Yes!
Very good correlation in Scripture for the same event, which IMHO still won’t be the worst massacre of humanity.
I believe that happens at the end of the Millennium Kingdom when Satan deceives a huge portion of the world and convinces them to personally attack Christ Himself in Jerusalem starting in Rev 20:7-9.
Like the “sand on the seashore” is the description of the number of rebels who have the audacity to attack Christ. I picture billions. Again YMMV, that’s just MHO.