We acquired a small Lychnis chalcedonica several years ago from an end of year plant clear out somewhere and could not be more happy. It took a year to get going but throws up a cluster of bright scarlet flower heads that with the benefit of a Chelsea chop in May has run and run this summer. Being another that has a good height to it coming in at around a metre tall making it makes a perfect specimen for your mid-level planting. It’s also tough as old boots when it comes to the cold so you don’t have to worry about putting it in a cold spot.
The flowers heads are comprised of terminal clusters of brilliantly coloured star or rather cross shaped petals in domes about 10-12cm across. These dance about on the tall stems that branch and multiply from the base of the plant. They’re compact in their spread so I think are better planted several together, or maybe you’d like them all?