Yeah, I think we're entering peak Thiem. I don't see him getting much better, but he's really good right now and might earn better results as he gains confidence.
As for the generation thing, it is all very arbitrary, but I like the five-year spans of 1979-83 (Fed's Gen), 1984-88 (Nadalkovic), 1989-93 (Lost Gen), 1994-98 (Next Gen), and 1999-2003 (Whatever they are). Technically Thiem is at the very tail end of Lost Gen (born in Sept 1993), but given that these ranges are arbitrary, he might just represent the transitional player between the two. He also does seem to be the best player born in the range of 1992-95, maybe even all the way back to 1989, so he could also be seen as the "best of Lost Gen," or the player that bridged two eras.
He also was a bit of a late-bloomer, at least historically speaking, not finishing in the top 20 until 2015, the year he turned 22. So that kind of edges him more towards Next Gen, as NextGenners like Kyrgios, Pouille, and Zverev weren't far behind him.