What's your question? Ask

When a plant you don't want begins to take over, there's no easy way to get rid of it. Here are a few options: https://www.gardeningknowhow.com/ornamental/trees/tgen/wet-soil-trees.htm
After doing some research, I couldn't find anyone else who considered crimson flag to be a big problem. There were a few mentions of the little bulbs wanting to come up everywhere, but no one seemed to be really troubled by it. If you want to keep some of your crimson flag, you could corral it with root barriers so that it can't spread beyond its designated spots. These are mostly used for trees, but you should be able to find a smaller one that would be useful.
Of course, there's always another option - don't worry about the crimson flag, and add lots of other wild flowers and native grasses to it, to create a wildflower garden: https://www.gardeningknowhow.com/ornamental/flowers/fgen/the-wild-side-of-gardening-using-wildflowers-and-native-plants.htm