the step nobody writes down: dry the broccoli and cook the rice ahead. almost every soupy casserole goes that way, because the rice and the broccoli carry their own water in. and brown the flour in the butter before the milk goes in, or the sauce tastes like paste. hold some liquid back, add it only if it looks dry near the end.
my thing is walking kitchens, not cooking. so weigh it how you like. you won't ruin it. casseroles forgive.