Wellllllllllll... here's my regimen, and how I came to it.
Harrison's Bird Foods
I feed Harrison's, supplemented by fresh healthy treats. My first, and later, my current avian vet recommended it. My bird loves the pellets now, but to get him converted, my avian vet suggested putting pellets out all day, and putting seeds (his old diet) out for two 15-minute periods a day. That would sustain him but leave him hungry enough to try new stuff. I presume the same technique could be used to get him to eat other healthy stuff, like fruits and vegetables! My guy was eating pellets in a couple of days, and now I can feed a good variety of other stuff, knowing he has the pellets as a basic. Pellets are out all day... fresh treats a few times a day. I also like Harrison's via mail because I never have to worry about out-of-date products.
I've seen a few of your posts... you seem very motivated to care for your new cockatiel, and you're being confronted with a few challenges! I may have missed it if you mentioned your location. With all your concerns, I'd love to see you consult an avian vet. Here are some links.
Certified Avian Vets
https://abvp.com/animal-owners/find-an-abvp-specialist/
If none are near you...
Avian Veterinarians
http://www.aav.org/search/custom.asp?id=1803
In my opinion, any of the vets listed here should be better than a regular vet.