Buy Superstar Hero difficulty and go play Daily, Weekly Challenge and some missions like Comet Chase, Meteor Storm, Retro (if you find wormholes),… and remember to sell your food at Space Burger.
That’s how I do.
Squawk blocks reward tons of food that you can sell, boss rushes are pretty fun and the hard ones give a lot of keys, and daily & weekly challenges. I also do comet chases, supernova missions, and retro missions.
Yeah, the difficulty does matter. You should definitely try higher difficulties because they increase the key drop rate (I might be wrong, but I sure do get a lot more keys on Superstar Hero than on Rookie).