My experience has been that using average monster damage speeds up play a heck of a lot, and the cost (in terms of making combat more predictable and boring) is not significant.
So, I would go with option 3: Allow the players to use average damage instead of rolling. Of course, players have to declare this ahead of time--they can't roll damage and then take the average because they rolled badly! You'll also need to address things like Great Weapon style and Elemental Adept, which key off the damage roll.
So, I would go with option 3: Allow the players to use average damage instead of rolling. Of course, players have to declare this ahead of time--they can't roll damage and then take the average because they rolled badly! You'll also need to address things like Great Weapon style and Elemental Adept, which key off the damage roll.