Can a lost rooted/unlocked phone be decrypted by a thief?
If your phone is encrypted, then it cannot be decrypted without having access to the decrypt option in Settings. And since you have a fingerprint lock set up, as far as my knowledge goes, you are safe - no one will be able to access your data, even if you are rooted. Decrypting the phone externally should require extensive hacking algorithms and knowledge about the phone architecture, and I doubt anyone will spend that much time.

If, however, the thief wipes your phone (factory resets it) from TWRP, then all the data will be lost, which again means that your data will not be available to the thief, and you are still safe.