I would also vote for the palm rejection for the iPad Pro + Apple pencil. I’ve been using Pleco for many years, and switching to the Pencil 6 monts ago has been qualitatively different compared to using my finger for flash cards. First, I started developing repetitive strain issues with my finger (probably too many flash cards in a row!), and the pencil relieves that problem. I’m also improving my character penmanship, and am finding I can do this much better with the pencil. This is particularly helpful now that there are more fonts in Pleco that are good models for writing. These plusses outweigh the minuses of efficiency you mentioned and even the palm rejection issues.
Finally, the Apple Pencil has actually become fairly useful for the iPad, so my non-Pleco workflow has my pencil nearby, and I use my pencil for other apps as well. Unfortunately, without palm rejection, Pleco flash cards is actually functioning less well for me now (as my characters keep getting deleted when my palm touches the screen).
Thanks for all your wonderful work on Pleco—it’s the one app I rave about to all my friends and family!