Can I make a roux a day ahead of time when making étouffée

Make a buerre manier (equal parts butter and flour, mixed to a smooth consistency) and when cooked out the next day, it becomes a roux. So, yes, you can

You could also pre-toast the flour then make the roux as needed and it will go much more quickly - I do that for my gumbo:

https://food52.com/recipes/11428-all-day-duck-gumbo

Roux keeps very well. If you don't make it frequently, be careful as it may continue to darken once you turn off the heat. When using it immediately, you can stop the cooking by adding the chopped veg, but I have had roux go from peanut butter to dark chocolate in color by leaving it in the pan (but off of heat).

Sure can. You might consider making a double or triple batch and freezing the rest. The roux keeps in the freezer for many months, and it doesn't take much longer to make a big batch than a small one.