I've had quite a few hypothyroid dogs, but almost all of them were old(er) Shelties -- it seems to be almost a given that in middle age, the endocrine system in a spayed/neutered Sheltie will include a sluggish thyroid. It is occasionally hard to convince a vet of that, since a hypo Sheltie will still test in the 'normal' range, albeit on the low end; but the breed should be in the mid-range of the tests to be normal on thyroid.
Most often, I have doses with Soloxine, and have had no issues with that. For a couple of the dogs, I first tried Armour Natural, but in cases of definite hypothyroid, I didn't have success with that.
I don't think my experience is of much use in Zeke's case, since I have never dealt with thyroid condition manifesting in behavior. Good luck!
PS: For the dosing, my vets would always start with a fairly high dose, and re-test in 30 days and then calibrate the dose from there, often reducing it. I've usually done at least two follow-up blood tests to establish that we have the dog at the best dose.