Firebase security rules wildcard in get function - firebase

Question

I am struggling with an issue that is seemingly easy at first glance yet still does not work after trying dozens of different syntaxes. I've got a firebase database with a collection called users and every user has a subcollection 'posts'. What I want to achieve with implementing security rules is that one's posts can only be read by oneself if the account is private or by everyone if the account is public. Therefore, each user has got a boolean field isPublic. Sounds simple. This is my code:
match /users/{uid}{
function isPublic(){
return get(/databases/$(database)/documents/users/$(uid)).data.isPublic;
}
allow read, write: if request.auth.uid==uid
match /posts/{postid}{
allow read: if request.auth.uid==uid || isPublic();
}
The error is in line 3. If I hard code the uid (currently, I've got just 2 users) of user 2, who is public, and try to open his posts as user 1, it works fine. However, using the wildcard throws an exception every time.
return get(/databases/$(database)/documents/users/$(uid)).data.isPublic; //does not work
return get(/databases/$(database)/documents/users/uid).data.isPublic; //does not work
let userid = uid;
return get(/databases/$(database)/documents/users/$(userid)).data.isPublic; //does not work
let userid = string(uid);
return get(/databases/$(database)/documents/users/$(userid)).data.isPublic; //does not work
return get(/databases/$(database)/documents/users/ogfTdmsSG4dy2AHf8dp7LcPpd9B3).data.isPublic; //works
I would really appreciate I someone could tell me what the correct syntax is. Thank you very much in advance!
EDIT
The associated query that fails is:
_database.userCollection.document(idOfUser2).collection('posts').getDocuments();
It works fine when logged in as user 2 with security rules activated, and it works fine without security rules no matter of which user is making the request, but fails when user 1 is making the request with security rules activated.
Additionally, I should mention that if I use another function instead of get (for example, in line 5, like this:), it works fine as well.
allow read, write: if request.auth.uid==uid|| resource.data.isPublic
Thus, I came to the conclusion that the get command does not work. However, I need the get command as resource does not work in subcollections.