Allowing for one right angle turn is fine. Any more than that will reduce functionality and is not recommended. If you have any more questions feel free to ask!

Is the ceiling also the roof, or is there an attic in which duct directional changes can be made?

The ceiling is between two floors, and the rectangular duct currently runs horizontally between two joists to a side wall of the condo. The plan would be to have one right angle turn, go through a joist, then immediately change to a circular duct and descend into the range hood. So one more right angle than there was originally.

I hope you meant go through the space between joists. Cutting a joist would require a significant strengthening exercise around that cut.

The pressure loss of another right-angle turn may or may not dominate the overall pressure losses that reduce your blower's flow rate. We would need to know the entire system design as well as the make-up air scenario to make an estimate of the effect.

To my knowledge, given the diameter of exhaust ducting, you would not be able, by code, to cut through a floor/ceiling joist. However, I'm not a contractor so I would advise you to contact a reputable contractor about this idea.

How about something like this instead:

1) Replace the current 90° rectangle to rectangle elbow in the ceiling with a rectangle to circle elbow.

2) Run a circular duct at 45° from the ceiling about a foot, using a circular elbow cut in half at both ends (unless they make 45° elbows?).

3) Hide the run inside a drop ceiling or ceiling mounted cabinet.

Any obvious issues there, other than having to hide a length of duct somehow?