I love the different hotkeys, I just discovered the spacebar a month ago and it helps a lot when I meet 2 deceptively similar grammar points. There is just one function I use that I could not find a shortcut for: showing/hiding furigana.
Every now and then a word comes up that I don’t know or can’t remember the reading for, when this happens I have to lift one hand off and use the mouse to click said word. It’s a very minor but simple addition. It would enable the users who rely heavily on shortcuts to blast through reviews with nothing but the keyboard.
I would recommend the up arrow considering that Bunpro already uses the down arrow to show the notes and that furiganas are usually on top.