if on an Apple device definitely just use the Notes app and also from the app store download and install the Poker HH Keyboard (Poker hand history keyboard) app. It'll add a different keyboard for a quick way to write poker short hand notes without switching the keyboard a million times.
I was thinking about something like this before seeing your post, but for Android since I use Keep for my session notes.
Nothing like this currently exists in the Play Store, so I'm going to pop open Android Studio over the weekend and see if I can whip a prototype together.