https://www.acvd.org/
get to a dermatologist have a skin culture done as this sounds like staph to me and it also sounds to me like your dog due to age is hypothyroid so you need to have a full thyroid panel there are six values T4, T4FREE, T3, T3FREE, T4AA, T3AA
My dog has staph right now and it can be nasty - it takes 25 days on an antibiotic and the best one is simplicef as that usually kills it and you can bath with duoxo chlorhexedine shampoo as well to kill topically.
we go to a dermatologist and they know more than vets about skin conditions
another thing someone on here recommended that worked for another dog but due to mine having allergies i was too afraid is steeping chamomille tea let it cool then poor into a water jug and use distilled water - make sure cool water then do a rinse and leave on do not wash off as tea kills infections in the skin - another lady on here her dog had blisters on the back and it killed it fast - i was to nervous as my girl is allergic to alot of stuff
I really do not think this is mites bc if it was sarcoptic mange you would get it and if demodex your dog would have had that since a pup since it comes from the mama dog