Yah it is kind of the nuclear option when there are very few mites. It is very harsh on the bees but is effective. I like OAV in the spring before supers just to keep the population nailed down. Formic toward the end of the season before winter preps right before the mite population tends to explode.I would not treat with a queen excluder on, but then I don't use them anyway. For a few hours you want the queen to be able to get far away from that formic acid vapor.
I'm also not a fan of spring formic acid treatments. Mites are rarely an issue in the spring when the hive is building up fast and a couple local beekeeping friends have lost hives after spring formic acid treatments.
I have good luck with a single treatment in late August or early September, soon as it's cool enough. Zero lost hives last year.