I don’t see how you can think this without applying some properties to that first mover. Even when talking bout this you say “is God” or even gravity you’re assuming that it’s still around and wasn’t say destroyed in the process of creating the universe.

Which is kind of my point. Even just the word “is” is quite a large assumption that is easy to overlook. I think it’s an outgrowth of saying some thing created existence rather than saying some event created existence.

Edit: Even thinking in terms of assumptions it’s hard to list them all. We think of time as a linear progression which naturally suggests a first action. But set that aside and really strange possibilities show up. What if the universe was created at the end by something that’s yet to exist and time flowed backwards from that event? Now it’s last action that created existence. Or even in the middle with a positive time universe moving forward and a negative time universe moving backwards. Or…