Even if you turned it back at this point, it still wouldn’t work.
This is pretty infuriating though; Google works just fine with any device that doesn’t run Android so why would they care that you’re running a custom ROM?
My guess is something less evil and more mundane: something about your number changed in their system and now they can’t send codes to it, which is why it’s grayed out. Maybe it was previously classified as a mobile number but now is classified as a landline.
Your only option, if you don’t have any backup codes, is to use that “Get Help” option they have that takes a few days and then either start carrying around backup codes, a Yubikey, or De-Google.
Hey, maybe all 3!
Make sure you’re on 1.19.0 of F-Droid, which needs to be installed manually for now.
https://f-droid.org/de/2024/02/01/twif.html