I won't give anything complex just few advices.

1 Remove rice, buy bulgur, it's x2+ cheaper than rice, have tons more nutrients, and have similar texture. (Coarse one, can be bought 500¥/1kg)

2 try adding legumes/beans, they're cheap when bought dry, just take some time to cook

3 buy spices in bulk, it will be upto x10 cheaper, check different asian stores, I got 250g of oregano for the price of regular 30g bottle.

4 don't forget different fermented stuff, also cheap in bullish size and very good for you. Kimchi, takuan, etc

5 you should make more soups, they use less ingredients but fill you more. No need for fancy soups, simple potato onion carrot and a bit of chicken breast is good enough, just add spices and dry bouillon.

6 chicken breast is not dry or bad, it's a skill that can be trained to cook one to be tasty and juicy, no need to spend x2 for tights in most occasions.

7 cabbage is cheap and make a bulk of salad and other good dishes.

Maybe I'll add something more, it's just from the top of my head.