RE: What more could he gain?
He needs more soldiers ( young men ) to throw into battle. I was in Eastern Europe last week and I saw a lot of Russian men who fled Russia to avoid conscription. What better way to justify conscription than to drum up patriotism?
Most of the world have been saying that Russia is the aggressor in this war and Ukraine is the victim. Well, a little terror attack will give Putin the propaganda tool to respond that this is not so. “I am protecting Russia from those Ukrainian terrorists”.
You can speculate and I can speculate.
I doubt most of the world is saying that. They aren’t that stupid nor is their memory that short. While many in the west may believe that the rest of the world saw the US/UK and NATO role in starting the war. While you were traveling did you happen to ask their opinion of the US and NATO and the EU?