Loose leash walking takes a looooong time to train for some dogs. Niko figured it out within about three or four months but it took a LOT of treats (mine are homemade, Google it for recipes and save $$$). Now he's about 80% loose leash, but he forgets now and then if he sees an animal or something worth sniffing. I have never used anything but a flat collar on him.
Miss Rosa, however, has made nearly zero progress over the past year using this method. If she weren't such a "soft" dog, I might consider something other than a flat collar for her. But she shuts down so easily, we are just going to keep with our method (stop when she pulls, start when she sits).
I'd suggest you follow the above advice and look at some of the other devices out there. Many, many people use a prong. That's not one I'd choose for my dogs but only because Niko's already almost got it, and Rosa couldn't handle it.
If it takes a treat to get him to do a command, it's better to use a treat than to repeat the command (he learns to ignore you). Like a trainer told us, "Would you work for free?"
You can ease off the alpha thing too, if you use NILIF. And it seems kinda strict to make your dog sit to enter/exit rooms IN your house. We used to try to leave the house ahead of the dogs, but gosh that's inconvenient! Now all they have to do is sit, and wait for our release word, and they may exit. I feel without a doubt that both dogs understand that my husband and I are firmly in charge.
You sound like you are off on the right foot though!!! Patience, your dog is still adjusting. Give it a couple more weeks, keep things mellow, strengthen the bond. And classes ARE a great idea. Plus it's fun to meet other dog people!
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old