I have 2 dogs at the moment.
One is a golden doodle that would eat anything that didn’t eat him first! Seems labs are like that.
The other is a chow/Shepard mix. He was an odd eater from the time I found him as a small puppy. He would nibble and guard his dish from the other adult dogs, yet allow any baby animal, new puppy or kittens to eat with him.
He seldom finished his food. And has always been lean.
He also has other oddities, such as wanting to sleep in a den of sorts, like a laundry basket in the closet, under furniture, in the laundry room.
As he aged, his odd feeding behaviors got worse. We made a few vet visits, because he was and is alarmingly thin. Many tests run, and we talked about an appetite stimulant. Like the OP, we tried MANY different foods. He eats something well for a few days, then won’t eat. And no, dogs like this won’t get hungry enough to eat what’s in front of them. Literally, if anyone saw this dog, they would turn me in for starving him!
On our last vet visit, vet said at this point, feed whatever he will eat. And that’s what we do. I TRY to get some actual high calorieDOG food into him, usually canned. But if he won’t eat that, I will scramble eggs with ham and cheese, cook chicken, hamburger, anything to get him to eat.
Is it a pain in the ass? Well, yeah, but being a Mom is not always easy:). He’s 15 now, and though he is thin he is still lively and happy. So we will keep babying him.