Desinif, are you open to getting this dog off of kibble and feeding an alternative diet? I have one of these dogs who lost the genetic health lottery too, and I think getting the dog off kibble probably saved his GI tract.
Instead of fooling with other kibbles, I would try transitioning him to The Honest Kitchen's Preference (grain-free) base mix -- plus whatever kind of meat he does well on (raw or cooked). You can even use eggs for some of the protein.
Since he's been on fish, maybe try beef or cook up some ground pork and pour off the fat if pork agrees with him. The base-mix is designed to make a complete meal with the addition of any fresh meat, so you have control. You can easily control the fat content through the meat if you need a lower-fat diet. The box tells you the ratio of base mix and meat for a given weight (though mine eats more than the box says to hold his weight).
I really believe that this particular food has an anti-inflammatory effect in the GI tract--I think it's possibly from the quercitin-containing fruit/veg, but the greens probably help too. It also produces HUGE (mega) poops -- and that's GOOD for dogs with anal issues because it empties everything out naturally. You'll sometimes see chunks of veg in them -- don't worry about it.
My dog had the constant butt licking, red, inflamed anus, impacted anal glands, and eventually progressed to bloody diarrhea. He was on multiple rounds of antibiotics. He had a surgery to remove polyps in the rectum caused by constant inflammation. His GI tract was "on fire." We had to apply topical meds to try to bring down the "heat," but it didn't help on the inside. He was miserable. Emptying his anal glands at the vet was incredibly painful for him because everything back there hurt. The vet thought we were headed to progression into PF, and possibly poor quality of life -- we had a very serious heart to heart about his concerns about the long-term prognosis. He had a hypothesis that the whole thing had a food allergy at its root -- if we could turn off the inflamed GI tract, everything else might calm down and then things might function normally again.
We tried a bunch of different Limited Ingredient foods, different brands and proteins for 6 weeks each -- it takes that long on each food to do an elimination diet. He got a little better on some, but never turned around completely. Something in all of the kibbles was still not working for him.
My last step out of desperation was to get him off kibble entirely and do something radical -- that was what led us to THK's Preference and beef. Within 2 weeks, the licking stopped, his anus went from red and irritated to pink and healthy, his poop firmed up, and his stinky anal gland smell went away because they were emptying naturally. I got my dog back.
We challenged him a year later with Acana singles beef kibble since it has ingredients that are similar to what he was eating and tolerating. We went right back to problems. I can't explain why -- I'm convinced this dog just can't eat kibble. The owner of a little independent pet food store I frequent has a Dane that's similar -- she has access to all the kibble in the world, and he gets sick eating all of them....the only thing he does well on is a raw diet.
I've tried other base mixes (including one much more expensive), and my guy doesn't do as well. There's something about the bulk of THK stools that really makes a big difference.
I add a Taurine capsule daily because THK has none added. It's especially important if you feed meat that has little in it naturally, but it's easy to supplement. We also add fish oil.
Glandex may help too. https://www.glandex.com/collections/glandex-for-dogs