Hookworms/Larval leak

Please help

Guys I seriously need help, like I’m to the point of losing my mind. I got my (currently 5 month old) standard poodle from a breeder 2 months ago. Within the first week of having him, I took him to the vet and found out he has hookworms. Fast forward to now, we’ve gone through 4 rounds of treatments. 1 was a pill I don’t remember, the second round was 3 doses of panacur, the third round was 2 doses of drontal plus, and then the 4th treatment was 5 doses of a stronger panacur. He still has hookworms, and my vet is saying that she thinks it may be larval leak syndrome. She’s saying I should try advantage multi (bc of the moxidectin) to see if it would work. She says itll treat both the larval leak and intestinal hookworms…Im at a loss as to why this wasnt suggested after 2 rounds of treatment, why make us go through 4 rounds? On top of that, they never gave me antidiarrhea until my dog had a really bad diarrhea episode after the dose of drontal plus. My dog still hasnt had a solid poop since august 26th. My family and i have been keeping up with cleaning up his poop immediately and sprinkling diatomaceous earth food grade (what the vet recommended) where he poops. I am so incredibly frustrated at the breeder for selling dogs that clearly have worms and I’m frustrated at my vet because nothing is working. Is my veterinarian incompetent? I’ve had a lot of people tell me I should find a new vet. Is there something I could be doing wrong? I’m doing exactly what they told me, having him poop on a contained area, picking it up immediately, and sprinkling the powder on it