Difficulty Working with Koji

I’ve made a shio koji “starter” using koji-inoculated rice grains, salt, and water, and have added fresh rice to it once in a while to keep it living and fed. I’ve heard many chefs and home cooks talk about the value of koji’s proteases and amylases for tenderizing and increasing glutamate concentration in meats, but I’ve run into a problem when marinating chicken thighs in shio koji. While pubs like Cook’s Science advocate anywhere up to 3 days in a shio koji marinade, I find the salt content generally recommended for use in shio koji just ends up curing the chicken, and you get a very stringy, unpleasant texture in the final result when cooked. Has anyone had this experience? Are shorter marinades the way to go? Reducing the salt content?