Dashi has no soy sauce in it. What you want is to make a good dashi, and add your desired soy sauce to it. Anything short of that will be sub-par. Dashi is so ubiquitous that coming short of making your own is tantamount to malpractice
Look for good recipes and processes for making dashi. At a minimum, you need to be able to find kombu, a kind of edible kelp. Broth made from that is the minimum. Then you step up to include katsuobushi (bonito shavings). Then, for a serious step up, you can add dehdrated shiitake mushrooms to the mix.
It is not hard, and does not take long. There is a process, so study, then do. You won't be disappointed.
The ingredients are available at Asian food stores, or Amazon, and keep almost forever, being dehydrated.
The soy sauce I use is "Healthy Boy" brand "Mushroom Infused Soy Sauce." It is not Japanese, per se, but the best I have found for Asian cooking.
Get on Amazon and look up each ingredient, then go on YouTube, and see how to get the job done.
Enjoy,
Rich P