1: “First mover advantage” belongs to the first one who gets it right, not the first one to try.

2: You missed the point. Which is: sorry, you’re almost certainly not the first mover already. Whatever it is, it’s pretty much guaranteed someone already tried (and, if you never heard of them, probably failed).

3: What you lose in networking and feedback by keeping your idea secret is too much of a cost to pay for offsetting the almost negligible risk that someone you talk to will go off and implement the same thing better or earlier than you.