The Mithras cult did not have any particular attachment to trees, you are probably confusing it with Saturnalia. But even then people did not bring a single solitary tree in, Saturnalia was celebrated by decorating rooms with many evergreen plants and branches as was common in a lot of winter solstice celebrations. The act of bringing in a single evergreen tree and decorating it came much much later, medieval times.

The Mithras cult gets far too much credit for these things. We know very little about them. Almost everything you’ve heard about them was made up.

The modern Christmas tree tradition goes back a few hundred years to central Europe.

Same with the Easter bunny.

I thought it was the Norse thing. But that might have been the stockings