(assuming 5e or newer) You can’t true polymorph into Tiamat. The ‘aspect of Tiamat’ from Fizban’s is already CR 30, Tiamat herself is unstatted but obviously more powerful.
As a player character, you can only True Polymorph to a creature of CR 20 or lower
So far, I’ve played three sessions with three different groups. Haven’t run into big issues yet. Next session I will be using monsters from the new MM. I’m curious to see how the balancing turns out.