Looking for design advice for custom electronic lock
Website says nothing about rating for holding it open, but if it's a solenoid that has the moving core seating against the stationary when energized it won't take a lot of current to keep it there (since the magnetic circuit's reluctance drops a lot unless there's still an intentional airgap at full travel).

You could just PWM a single FET to drive the solenoid, it likely only takes a 50-100ms pulse at full current to move the mechanism, then you can drop the duty down to something low to keep the current at a value that'll hold the solenoid seated. During the off period the current circulates through the freewheel diode, the decay is usually quite slow, so the PWM frequency won't need to be more than a few hundred Hz. No need for the step-down.