Ordered a 512GB version of the 12.9 last night (along with the 2nd gen pencil) and it's being delivered tomorrow, upgrading from a 2017 10.5 64GB iPad Pro and can't wait
Exactly my situation (+ magic keyboard), you won’t regret it! The larger screen size is a bit of an adjustment but I’ve gotten used to it and I really enjoy the extra screen real estate, especially for note taking! It has actually changed my entire productivity workflow.
For times where I’m in the mood for a smaller screen I still have my 10.5 which for all intents and purposes is still a great device.