Multiplayer server with firebase and unity for an indie game?

I am developing my first indie mobile game using Unity3d, I have integrated google firebase (firestore) to store game & user profile data, also I am using firebase user login service, currently I am using its free tier.
Now I am planning to make this as a multiplayer game, I came across multiple choices such as google cloud, aws gamelift, photon etc.
But I am not sure which one would be a best choice, following are my priorities:

  1. It should have a free tier to handle initial testing load of a couple of CCU.
  2. It should be scalable.
  3. it should be compatible with firebase.