I do the math, but I also go with what tastes and feels good.
I guess legumes could work well as protein source, but I don't digest them well and fart all the time :D
I have something with beans or lentils once per week.

Probably not common where you live, but different organ-meats have excellent nutritional values and are sometimes quite cheap. (liver, lung, bones (for soups))

I get my first 600-800 calories in by just drinking heavy cream in my coffee.

Then again, I work physically and need much more than 2000 cal/day.

Nutrition is a complicated topic and kinda depends on your lifestyle, location, gut-biome, genetics, etc...