There are several reasons that I’ve found can make these pajeon mushy/soft. First, if you don’t pre-saute the mushrooms, they leak liquid into the batter while you cook the pancake and stop it from crisping up. Also, you MUST make sure the pan and oil in the pan are hot enough. You can test this by adding just a drop of batter to the pan, to begin with. It should sizzle as soon as it hits the pan. Adding a little extra oil may also help, but I try to avoid adding more than necessary. Lastly, the pancake mixture being chilled before being added to the pan can help to yield the crispiest results. There’s no need to wait for it to rest in the refrigerator for 30 minutes either. You can simply use cold milk/water for the batter.