I wonder if you played some tiel recordings or whistlings at times during the day if it would help.

I'm with seesthru on the Harrisons. I don't see any reason why you can't put it on spaghetti as long as it's not left out for no more than a couple of hours. Also, Avicakes are very nutritious so if Max is eating them, that's good. The Lefeber website says that Avi-Cake nutrition matches that of most pelleted diets and can be fed as a main meal.

https://lafeber.com/pet-food/bird-food/avi-cakes/#:~:text=Cockatiel%20Three%20Pack%20With%20Free%20Tropical%20Fruit%20Pellet%20Sample&text=Your%20bird%20might%20pick%20and,fed%20as%20a%20main%20meal.

Cathy