On tapjoy don't do any of the surveys or the "sign up for a membership" offers; you will likely end up paying for a membership you'll never use, and possibly never get your cat food. If you want a large amount of cat food without grinding, buy the special offers because of their deals.
I always send my gamatoto and get 5 cat food on average, finish SOL stages for 30 cat food with different difficulties. Zombie stages are easy to farm 10 cat food. The weekly missions provide cat food too.
I could be wrong but I noticed when I have more XP than cat food, my gamatoto brings more cat food than XP and vice versa.
Best way is to do tons of SoL and events, Gamatoto is nice to have as a bonus and do Outbreaks whenever they appear. Tapjoy isn’t worth it and ads don’t work for me, but they might for you, and if so they’re kinda good to do while also doing something else to get some quick cf.