I've almost always had a dog throughout my entire life. I've had dogs that ate most foods when offered and then others who vacuumed up non food items like they were living garbage disposals.
My current dog was a rescue from a shelter that closed down right before the COVID lockdown. He had a really rough life and had spent most of it living in the shelter after being abandoned in the woods.
He's my little cPTSD buddy and I love him with all of my heart, but my god this dog is the pickiest eater I've ever seen. He's not feeling well these days, (which definitely doesn't help), but to a certain extent he's always been this way since I brought him home.
When he first came to live with us, I asked what kind of food we should get him. They said he was used to just eating the cheapest kibble, so it didn't matter. We still had some of the fairly pricey dog food leftover that we had been feeding our previous dog. She had loved it, so we figured it would be fine.
Almost the entire first week he lived with us, he just wouldn't eat anything except table scraps.
We switched to a different kibble and that worked for a while, but eventually he just stopped eating until we switched to something else. Even wet dog food is hard to get him to eat if it's been in the rotation for too long.
Not only does he just flat out refuse most dog food after a certain point, if he finds we're trying to sneak medicine in human food he likes, even once, he will refuse to ever eat it again.
I'm trying to get him to gain some weight. So far he has eaten for a while and then gotten sick of: chicken, hamburger, bacon, and most recently rice cooked in bacon fat (we're a pescatarian house, so keep in mind this is all being cooked exclusively for him)..
The rice in bacon fat seemed like a big hit until I made the mistake of mixing in a teaspoon of pumpkin (which he also liked at one point) into it.
As of now the only thing he will eat seems to be grilled shrimp. :(
We switched over to a raw diet maybe about 5 years ago. It's shocking how much more energy our dogs had, how much healthier they looked, and how small their poops got with less frequency. Kibble is mostly just waste with calories since a lot of the nutrients are baked out. With the raw food, they absorb a lot more so less waste comes out. It's definitely more expensive, but I don't plan to ever go to something else after having seen the benefits first hand.
We just adopted another rescue after our last boy passed in January. The shelter feeds all of their dogs the same kibble to keep things simple and not needing to deal with changing proteins and the mess that often accompanies it. Our long term dogs poops once or twice a day, doesn't stink, and almost her poops almost look like cat poop. She's a 70lb husky/pitbull/German shepherd mix for perspective. The new boy is a golden retriever/doberman mix per the people that surrendered him, but he looks more like he's a great dane in a big labs body. He poops 3-4 times a day, you can smell it from the neighbor's, and you need a small crane to pick it up. I just started the transition from kibble today. Oh, they also drink way less water because there's so much more moisture in the raw food vs super dry kibble, even though we keep full water dishes out for them at all times.
None of the above really addresses your topic, but raw food might be something to look into. The raw food we use is Raw Advantage. It's messier to prepare and kind of stinks like farts because of some of the stuff in it, but it's absolutely worth it imo.
We had a Rottweiler years ago that had to have both ACLs repaired and she stopped eating certain foods, so we went through a similar situation of swapping stuff until she would stop eating it. If she ever tasted the pain pills we tried to sneak in, she wouldn't even entertain the idea of eating that type of food again. All trust was gone. For pills, we've found wrapping them in a little bit of American cheese works great. If they are picky already, shoving it to the back of their mouth generally works and is easier wrapped in cheese instead of the raw pill.
Best of luck with your pupper.