I'll answer your questions with a bit of a discourse. When I moved here about twenty years ago, I put the fields into Countryside Stewardship in exchange for a capital grant on a pond project (the money was easier to get then). The fields had conservation interest with just under forty odd grass and herb species identified at survey. I wasn't farming myself at the time so outside graziers used the land and I was supposed to keep the graziers to a regime intended to conserve the fields as hay meadows which I know historically they undoubtedly were. All well and good but over time it proved a struggle as inevitably the graziers would bring on more stock than the fields could cope with, half the time they couldn't be bothered to make the hay and, perhaps worse of all, would bring in supplementary, non-indigenous feed that introduced masses of weed species. I ended up hand-pulling well over ten thousand thistles from one field alone. It made me resolute to turn it around so I decided to become a farmer myself and learn from scratch.
Turning to your points. All the species in the field are worth conserving, including the odd creeping thistle. It's the balance that needs to change as grasses and rank species dominated and thus need management to allow the flowers to show in greater number. There was nettle interspersed everywhere as well which is a tell-tale sign of enrichment. So I've re-introduced a management system based on hay cuts and managed to make about 400 small bales this summer, a year which has been fantastic for grass, I'm sure you agree. My fields are a shining example, or should be, of what an old English hay meadow once looked like. At the peak before the cut this year one field literally abounded with zillions of creatures that jumped aside as you walked through. It's an amazing sight. When you compare that with the productive but boring and silent rye grass monocultures that surround you, you'd start to understand where the motivation comes from.
Species-wise, there's scope I'm sure to do more but I don't particularly want to spend hundreds on wildflower mixtures where the prognosis is unknown. I have some hay rattle seed I've collected and I've bought some betony seed too which appears absent presently, but that's about it for the moment. I'm just trying to get a system of animals, grass and hay all working together to attain a kind of serendipitous harmony. It looks from your collective comments though that I may have overcooked the animal numbers a bit.