I've had a rhubarb saison before from someone in my brew club and it was exceptional! I agree with majority and say forget the rye. I wouldn't attempt to make a rhubarb syrup, I would simply start with a smaller amount and then pull a sample. If you want more tartness, add more rhubarb. I will say that it dri items added predominant tart characters, but it wasn't out of style. That's also controllable by the amount you use. Good luck!