Thanks for all the helpful feedback in this thread. I’ve released RC2.
You can see the issues that RC2 covers (here in the Done column): 9.0.0RC To Do · GitHub
I’m sure there will be much more to fix in the coming days, but hopefully this is helpful as you continue to work with the release candidate. Updating RC1 to RC2 should be a very painless process.
Additionally, I’ve started a tutorial detailing all the common pain points when updating add-ons/themes for version 9. I’m sure there is much more to add to this but hopefully this is a helpful start: Add-On Developers: Get Your Add-Ons Ready for Concrete CMS 9.0 :: Concrete CMS