This link says the winner of the last group wins the tournament. So for the tournament you describe, I believe the second round is the last.
Because there are no other groups to merge with, the tournament ends with that round.
https://support.chess.com/customer/portal/articles/1444838-how-does-a-round-robin-knock-out-tournament-work-
Hi
I'm currently playing a tournament which started with 15 players in 3 groups. The setting is 6 players per group, 2 advance.
So now we are in the second round with 6 players in just one group. Will this be the final round, or will there bere a third round with just two players?