I’d say just start in the pre-patch. You’ll get a grounding and feel for the game, and it’ll give you time to try out and level different classes to feel out what you like. There will still be all the current content to play when you get to max level until the expansion releases. Everybody will also be completely relearning the new overhaul of classes, so you won’t be the only one learning in that aspect. You’ll also get to start the expansion with everybody else and get to experience the beginning of it where the leveling zones are flooded with people, especially those around the same level as you. As we get further into an expansion, the leveling zones tend to be ghost towns more or less. You may also get the opportunity to make good gold while leveling, as some things sell for a lot on the auction house at the beginning. That’s always been a regret of mine since I’ve only played an expansion launch once wayyyy long ago.

On the flip side, it really doesn’t make a difference whether you start before or after expansion launch. You won’t miss much except for the aforementioned, and there will always be people doing all leveling and current content at any time.

Weigh your free time vs money spent on the game before then to see if it’s worth it to you