@dankcushions Switching the resolution to Mode 16 (1080p 60fps) in RetroPie seems to have resolved the remaining issue. The game takes up much more of the screen and now looks visibly correct while paired with using "Core Provided" for Aspect Ratio in RetroArch.
Thanks for your help, all! I'm still curious why my display settings got all changed around after upgrading to RetroPie 4.7.20, but I'll leave that to people smarter than me to figure out. :)
Happy classic gaming, everyone! ^_^