You need to burn or mow a meadow periodically to maintain the health and diversity of the open vista. Don't consider the mow disruptive - its not.

A prescribed burn would be very useful. Burning is very selective at controlling non-natives and is stimulating to the native flora.

Glphosate would work on the multi-flora. You can also spot-treat the thistle, hops and crown vetch with it, too. Joe Pye is right...a burn would also help with everything else.

Mowing doesn't usually kill a plant. It just makes really short, and it will bloom anyway. I had some 2" tall Queen Anne's Lace in full bloom this year, LOL! April

Burning might be a good idea but it isn't always easy to do in a relatively civilized setting such as a meadow within a subdivision. You could also try mowing, or perhaps selective mowing. If the thistles, vetch, or other weeds are concentrated in a particular area, mow that area early in the summer then walk around applying Roundup to the thistles. If you can't identify the thistles right after mowing you could wait a couple of weeks until they grow new leaves that allow you to identify them. This will take a number of repeat applications, but eventually you should be able to get the weeds under control this way.