Sorry to hear it, but not surprised. I hope the formic pro does the trick. I don’t have experience with it so make sure you do whatever follow up is necessary including testing to see how it’s working. High numbers of mites at this time of year will put a even a bustling colony at risk of winter die off, because the bees don’t have as much stored fat as they need due to mites sucking it away 
There’s a ton of info here in the Pests and Diseases section, here’s just one post: