just as, as though, fortunately
恰 centers on a sense of fitting coincidence: something happening just as expected, as though by design, or turning out fortunately.
恰 combines 忄 (heart/mind) with 合 (fit, join), suggesting a mental state of things fitting together appropriately. The exact historical development is uncertain.
The heart 忄 beside 合 (fit) paints a picture of a feeling that everything fits perfectly: just as you'd hoped, as though it were meant to be, a fortunate coincidence.
For コウ, imagine a coincidence so perfect it feels like a ko-incidence: ko -> コウ, and your heart knows it's just as it should be.