Hello,
You can still achieve your goal of having separate “instances” in a few ways. The most basic is to create another project and have one of the Firestore instances in that second project. You can connect multiple Firestore instances to a single Firebase app [1]. Another option is to use sub-collections within a single Firestore instance, as described here [2].
Another option is to use Realtime Database instead, since it allows you to have multiple databases per project, but it has some limitations around scalability compared to Firestore. You can see a more detailed comparison here [3].
Here [4] is a short description of subcollections from our public documentation. But I think this video [5] might be more helpful at describing when you would use subcollections, so you can compare with your use case.
[1] https://firebase.google.com/docs/projects/multiprojects
[2] https://groups.google.com/g/firebase-talk/c/dOs2IOOkg04/m/-rSSadkIAwAJ
[3] https://firebase.google.com/docs/database/rtdb-vs-firestore
[4] https://firebase.google.com/docs/firestore/data-model#subcollections
[5] https://youtu.be/o7d5Zeic63s