Definitely mites, and a heavy infestation if there are so many visible. Sorry about that.
Your OAV treatment regime doesn’t appear to be quite what is usually recommended. Treatments are meant to be 5 or 6 days apart (no longer than that) to interrupt the mite life cycle. It is optimal to do an “accelerated mite drop” count exactly 24 hours after the last treatment, and continue at 5 day intervals until it is below 25 mites per 24 hours on the sticky board.
I am impressed that you looked into it. You have the mind of a true professional. So sorry for your loss. I hate Varroa with a passion. We are about to start retreating our hives - early, but I see worrying signs. We have to stay vigilant, unfortunately.