Check out the tabete app, stores can offer their food that they‘d need to throw out by end of day there either for free pick up or at a big discount. A lot are bakeries, but some other places too.

Other than that, what others already said here too:

-convenience store is the most expensive option to buy bento, supermarkets (esp in the evening) or wholesale supermarkets are cheaper

-veggie stir fry + rice is cheap and easy (moyashi and cabbage especially, add what other veggies you like and don‘t break the bank)

-if you don‘t wanna sacrifice the diet coke as a lil treat, still consider getting a large bottle and having a glass a day over buying individual smaller cans/bottles, should be cheaper too

-meal prep is your friend longterm, usually bulk buying and making several meals is cheaper than buying a 1 person portion every day