Hey, thanks for playing the game! You should be able to hold down the LMB to fire attacks and as far as I know that wont trigger the power up selection. Both tapping & holding have the same cooldown.
The powerups are activated based on what your dice lands on. Each powerup is mapped to one of the six numbers. The original plan was to actually show the dice landing on a number in game but we unfortunately ran out of time to do so.
And yeah I can definitely add space to select as well after we can edit the game