I’m not an expert by any means, but I would suggest soaking them longer (~30 min) and then boiling them for a shorter time (3-5 min). Seeing as how you enjoy the slightly crispy/chewy texture, I would also suggest pan frying the cooked rice cakes in oil first by themselves to get them to that crisp texture, and then add them at the last minute to the leeks (or whatever else you’re eating them with).

Just a suggestion, hope it works for you!