Initially, I bought a few commercially available dashi soy sauce making kits.
Naturally, I poured soy sauce into them and used them, but what do you do with the leftover ingredients afterwards?
I crushed the bonito, shiitake mushrooms, kombu seaweed, and other ingredients soaked in soy sauce and used them as gyoza filling.
Since they are already well-seasoned, I just need to be careful with the seasoning adjustment.
But so far, the best way I've found is to put marinated dried young sardines on top of cold tofu (hiyayakko).
A side note
When I consulted a former craftsman in my neighborhood, he said, even before I finished explaining how to change the flavor of soy sauce, "I'd want to take out the marinated dried young sardines and mix them with rice to eat."
For some reason, we both thought of dried young sardines, so that's another way to use them.
Using dried preserved ingredients to create flavor in preservative soy sauce, and finally utilizing even the dashi-gara (leftover ingredients)? This is truly a waste-free way of eating.