Our oldgirl had chronic ear problems due to malassezia pachydermatis which is basically an overgrowth of yeast.
Like you we tried everything, spent ££££s on vet fees. The only thing that helped was putting her on a bones and raw food diet, avoiding all grains - but even then it didn't completely go and she would have regular flare ups and infections. In the end it was managed by the vet keeping us stocked with Surolan to use at the first sign of an ear flare up.
She was also atopic so had multiple allergies that we had to try and identify and manage, as if an allergy flared up, it inflamed the MP.
She was a shepherd cross collie with stand-up ears, but ended up with an aural haematoma from shaking and scratching and had to have it operated on, resulting in her having one ear up/one ear down from then on.
She also had MP around her toes and back end and just before she died we discovered Organic Neem oil. I was really sceptical, having been told about so many different natural remedies over the years and in fact put off trying it for the longest time, but when I did we were amazed. It absolutely stinks - worse than the yeast to be honest, but from the second application she stopped scratching and within a fortnight the fur was starting to grow back on her paws. It also gradually reduced the black greasy patch on her lower abdomen and drastically improved her external ear canal, although I was never brave enough to try drops actually in her ear and we lost her to cancer not long after we discovered the treatment.
You can buy it online, but need to make sure it's high quality and preferably organic neem. We bought some cream for her feet, pinna and abdomen, plus some concentrated drops to add to the cream for a boost and to use neat on particularly bad areas. We bought from here and got this this and this. The neat oil stinks, but the cream and shampoo don't.
Other than that, I have read several studies about systemic veterinary treatment of chronic fungal infections using 'azoles' - basically the group of medicines we would use for thrush or athletes foot. The dogs are given it orally to clear the whole system of the yeast. I mentioned it to my vet and he poo-poed the idea, but there are lots of peer reviewed research studies available online, so it might be worth discussing with your vet - or asking for a referral to somewhere that will be more open. Iirc you need to have cultures done to check which type of azole your dog's yeast infection is most likely to respond to.
Just a quick Google threw up this, which is an easy read.