From the sound of things, I definitely think roux is the best way to go.
I’d roux it as soon as you get the flavor profile you want.
It should hold up.
If everything is done right, you’d only have to reheat it, but if it is too thick you can thin it out with stock.
You sound like you have a good plan and know what you are doing, so just trust your instincts and you’ll be good.
Well, gravy came out decent, I used about 14 tablespoons of roux (had way too much stock) so I threw in 3 packs of (icky) pork gravy mix and some salt/seasonings (didn’t have any spare time to mess with it)- and it came out good - a thin sauce for the pork loin - had that sucker on the stove for 4 hours as everything else was getting done.
The caramelized onions and raisins was the hit of the dinner, excellent reviews. And nothing says banquet like those canned potatoes that we baked off and doctored with the pan drippings, butter and seasonings - another hit.
Pork loins came out of the oven at 140’ to rest - perfect.