I’ve lived with rushes (and reshes as they’re called in Cumbria) for fifteen years or more, and can’t say that the frost thing sounds at all likely, sorry.
To reduce vigour, top - hard - after the ground nesting birds are all fledged (so, late July in most areas) and again 6 weeks later. If you can get Fell or Exmoor ponies (or other conservation-grazing types of native ponies) on after that, that’s a big help. Some of the more primitive sheep can be useful too.
For wildlife, top 1/3 or the area each year, in irregular patches, so that there are varying lengths and densities, and open parts with shelter around about.