How do you properly set up Firestore Rules for Users to display username to other users?

I keep getting the above error when trying to load pages with usernames and I'm guessing it's due to updates to my Firestore Rules -- I swapped CRUD to Users Collection for Users.

Goal: Display usernames publicly but keep all other User collection data private -- e.g. keep 'email' field to read-only for 'Users collection' but adjust 'username' field to read-only for 'Authenticated users'.