I've never done the Chelsea chop but why not try it with a few Achillea and see how it does. That way you will get to lots of flowers.
At he moment I just have their feathery foliage no sign of flowers

Mine is already sending up flower spikes. Early on it's difficult to distinguish where the flower spikes will happen. I love the large arching silvery feathery foliage and it's elegant form. I wouldn't think of chopping it now. It will never look the same if it recovers.

The idea is that the plant sends up even more side shoots and then you get more flowers but later on.