9to5Mac has a thorough comparison of the new iPad Pro to its 2018 predecessor.
There are a few key under-the-hood improvements and additions, but the rest seems mostly like minor bumps.
Also, Appleās site confirms the new (obscenely priced) Magic Keyboard supports the 2018 Pros.
https://9to5mac.com/2020/03/19/new-2020-ipad-pro-vs-2018-ipad-pro-comparison/