Big oven roux questions

Provided you have a large enough cast iron pot, yeah you will be ok doing it all at once. Just stir it every 15-20 mins.

I've done 7 cup/7 cup a number of times in one pot. Takes a long time, but with very little attention until it actually starts getting dark. I forgot about it one time and got on a very very long phone call. Didn't stir at all for nearly 2 hours. No issue at all. It had been in about an hour already.

I've also started a large roux and run errands for over an hour or so after putting it in. It really doesn't need a lot of stirring until late in the process. I stir anyway when I'm not doing something else, but it browns on top and bottom in the oven.