The moderators might move this to home cooking... there's a lot of water in rhubarb and its not apparent when you cut it up. Let the sugar combined with the cut up rhubarb rest in the bowl for 10 minutes so the juice comes out then add your thickener (I like instant tapioca for rhubarb and strawberry-rhubarb). If you like cardamom I also think a tiny bit is a good complement to rhubarb so its good for adding to a crumb top. Cardamom is powerful though so 1/8 to 1/4 teaspoon is plenty.
Also, you might already know this but if your rhubarb comes with leaves, cut them off and don't eat them. They are poisonous.