I think Themberchaud will be the breakout meme of the movie. He's both terrifying and hilarious. My wife isn't a super sci-fi/fantasy fan and she enjoyed pudgy dragon the most of everything in the movie.
They do, but Szass is and always has been one of the most powerful and dangerous villains in The Realms. Unless somebody (not Edgin and crew, not even with Xenk) can completely destroy him he's functionally unstoppable over the long term. Not even the heavy hitters like the Seven Sisters and Elminster have managed to completely stop Szass Tam. The character like an archmage and a lich, and completely controls Thay. In 4E he was a Level 30 Artillery monster, in 5E he'd probably work out to be something like a CR28 to CR30 monster with a host of legendary actions.
Edit: On "Black Elminster" if I'm remembering right, Szass Tam did the take over Thay bit in the mid-1370s DR. The movie takes place in in the mid to late 1490s based on the events and apparent ages of characters like Lord Nevember. Elminster was born in 212 DR, so as of the movie he's nearly 1300 years old. Simon's great-great-great-grandfather could be Elminster's great-great-great-great-great-great-grandson. For some context, my greatx3 grandfather was born 1840s. Elminster would have been born when Vikings were still invading Anglo-Saxon England. And it isn't like the Elminster was chaste by any means, so there's lots and lots of time for Elminster to have lots and lots of descendants who don't share much with him in look department.