Yes, it’s very difficult to recognize young mushrooms. I think it’s a really nice way to think of mushrooms as mutable living things, and learning to recognize them at different points in their lifecycle (rather than comparing them to a random snapshot) is a lot of fun.

Is there a way to tell if a mushroom has fully formed?

For me it has been helpful to cut them open lengthwise to understand how the cap takes shape and opens up, rings are formed etc.

That said, some type of Armarillia seems like a plausible guess to me from the pictures.