best stucture for firestore collection

Hello, I have an Xcode app with a few thousand users and I use firestore to store my data. Every time a user registers firestore creates a random uid for them. Right now I have all my users in one big collection called "users." I was thinking about splitting up this collection into sub collections for each letter in the alphabet and then putting each user, based on the first letter of their name, in the corresponding sub collection. But the only problem with this is when I want to fetch the current user. When a user signs into the app I only have their uid from the firestore authentication. So if I want to fetch the current user I wouldn't really have any pointer to which sub collection to look into. I thought of maybe putting a pointer letter at the beginning of the UID but I don't think firestore allows me to change Uids. Does anyone know a better approach to this or how I can make this work. Id appreciate the help.