I did like that it focused on the mech, it gives the game a different feel to another survival wave game, that being said it is true, you do get the hang of it after a few goes. Will the mechanics also be getting more difficult as the player progresses in some way?
As for whether holding shift and the mouse being hard or exhausting; I didnt find it particularily difficult and personally I found it made the mech feel more interactive. If you let go of the button its like you're letting go of the Mech's sticks, it makes it more important, I would but up for trying a 'press to activate' but I have a feeling that it will feel too easy then, or at the very least a little less interactive.
Hope that helps!