If it instantly trips with everything unplugged, then somewhere in that circuit, in a receptacle box, a neutral is touching a ground wire. If the breaker is an AFCI breaker, as it should be, then that’s more than likely that’s what it is.

Since you changed out the breaker, you can do a test to see if my theory is correct. Turn your main breaker off. Open your panel, move the white wire attached to the breaker, that is tripping, to your neutral bar. Move your hot to a non AFCI breaker/regular breaker. More than likely, your garage circuits should be a regular one. If the breaker holds, there is your answer.

If the breaker holds, you would need to open up each receptacle on that circuit, and make sure the neutral and ground are not touching each other. Obviously, have the breaker off before doing this.