I’m in my 5th year and still experimenting on mite treatment. Since I only carry a few hives, my goal has always been 100% winter survival. If I had a dozen hives like the OP, I might cull a few which did not respond treatment.
I had a similar experience as the OP in 2016. I stubbornly continued OAV until December (until mite drop became zero), and ended up with one dead and two weak hives. Looking back, I should have switched to apivar instead (too cold for formic by then).
My most successful treatment was done in 2017, when I used formic when mite loads were still quite low in early August. The treatment worked well but one of the 2 hives showed sudden surge in mite drop in October (probably went robbing and got mites as well), so I treated both with Apivar which worked well, then finally OAV during winter brood break.
2015, 2 hives from packages, formic in September-October, OAV in winter, saw K-wings, 2/2 survival.
2016, 2 hives and 1 nuc, OAV series in August, continuous OAV from October - December, no K-wings, 2/3 survival.
2017, 2 hives, formic on Aug, apivar mid Oct - early Dec, no K-wings, OAV in January, 2/2 survival.
2018, 2 hives and 1 nuc, all got OAV after swarming in July, apivar Sep - Oct, no K-wings, OAV in December-January, 3/3 survival.
2019, 2 hives and 1 nuc, all got OAV during brood break in May or July, the nuc got apivar mid Aug - early Oct, two hives got formic (half dose x 2 times) in September and apivar starting in October. Seeing K-wings in the hives.