Just seen this and had a thought -
if you want to weaken the grass, then you will end up with thin 'hungry' grass, whatever time of year, may be OK for a bit of stock feeding, but wouldn't have the goodness the animals need, (some hay I bought last year is like this, something I'll be aware of in future), I think you have to decide which you want, personally I think I'd just have a smaller area for the wildflower meadow, and see how you go on with it for a year or 2.
I think you can get the flowers in plug plants to get started, more expensive but more reliable? at least you'd know what will survive.