Thank you! I chose the Web Colours Palette option. I then used TinyPNG on that exported image, just to see if it could be gotten any smaller. It actually was able to get it smaller, but just 11% smaller. I'm not too interested in diving through menus and options, so it's nice to know about the Palettised functionality. I also would prefer not to have to export into another app or website in order to optimize images. So, the Palette option seems to be the best option right now.
Thanks again!
(That said-- I'd love hear about anyone's experience with the TinyPNG plugin inside an Affinity app.)