Long-Term Skin Problems, Please Help!!

I adopted my hedgehog, Bean, at the end of January 2018 when he was around a year old. Once I started handling him, I noticed that his skin looked terrible: yellowy, flaky, etc. (see first pic). Also, he had significant quill loss in my first few months of having him- not as much as some hedgehogs will have, but enough that losing quills in the double-digits wouldn't be surprising.

I was really concerned about this when I got him, but I was hesitant to take him to the vet because I thought it could just be normal dry skin that would clear up with a better diet/shampoo/environment. Unfortunately, this never happened in the time that I've had him, but it's strange because there have been some periods of time where his skin looked a lot better before we even made the decision to take him to the vet (see second pic- his quills are very thin, but skin is much less flaky?). His skin never completely cleared up, but I still debated whether or not to take him to the vet, mostly because he didn't seem to be in pain,( though I did notice that he would eventually start getting some tiny scabs and redness around his quill line- presumably from itching).

Eventually, in August of last year, I decided that his condition did warrant a visit to the vet to confirm whether he had "normal" dry skin or not. The vet did a skin scrape while we were there and found one sickly mite, so we did a course of oral Ivomec and Metacam. I'm aware that Ivomec can be dangerous and would never be willing to risk it again, but the vet didn't carry Revolution and was convinced that Ivomec had a much higher success rate. After I communicated my concerns, he agreed to give Bean a smaller dosage for his weight, and I of course monitored him very closely.

Unfortunately, Bean's skin looked the same after this... After waiting for a month to see if the dry skin would heal (since I've heard it can take some time to go away after mite treatments), we decided to take him back to the vet. After giving him another skin scrape and finding only a few dead mites, the vet told us to keep doing only the Metacam for another week, which had no effect on the dryness of his skin. In December, we decided to bring Bean to the vet for a third time, and a different vet suggested that we try a round of Bactrim to rule out any bacterial infections. We agreed to her idea and tried the Bactrim for 10 days but still saw no improvement. The last 3 pictures attached are what his skin looks like now.

Since the Bactrim treatment, I've followed up with the vet by email to discuss further possibilities, and she said there are some less common causes of dry skin such as endocrine changes, cancer, inflammation, nutritional problems, etc., along with the more common mite/skin infections. So now, I'm still trying to decide what the best thing to do would be because his skin problems are driving me crazy! The possible courses of action I'm considering are...

1. Treating for a fungal infection: We could try this just to see if it helps, but I don't want to give him unnecessary medication just for the sake of ruling out possibilities.
2. Biopsy: During our last vet visit, the vet also suggested doing a biopsy, which would cost $300+. My biggest concern is that this is more invasive with a higher risk of complications/infections. Also, the vet mentioned that his issues might not be significant enough to go to such lengths, but the pro is that we would most likely know what the underlying problem is.
3. More skin scrapes/tests: This one is pretty self-explanatory. I just wouldn't want to do this and end up needing the biopsy anyways!
4. Try a different vet and see if they have Revolution: I'm considering that he might still have some mite issues, so we could always try treating for that with Revolution. I just think that his case is a little strange because he is still growing new quills and doesn't have any bald spots after over a year, only a general thinness of quills.
5. More "home remedies": I've tried flaxseed oil a few times in the past, but I've considered trying different oils or a humidifier. I'm just not convinced that he only has "normal" dry skin.

Other info:
His diet is a mix of three high quality cat foods: Chicken Soup for the Soul Weight & Mature, Solid Gold Winged Tiger, and Blue Buffalo Turkey and Potato. The only treat that he likes so far is plain cooked chicken (he even turns his nose up to mealworms!).
He gets a bath ~once a month, and I use the Aveeno oatmeal baby soap.
His cage temp is always ~73-77 degrees Fahrenheit, and his bedding is part plain Kaytee and part anti-pill fleece.

I would really appreciate any advice on what I should do, and sorry for such a long post! Thanks!