hi @eclairhola
"Am I correct that thumbnails specified in my feed are cached on Roku's side, and the Roku devices load (and potentially cache) thumbnails?" - from what i know and experienced , Roku caches the feed not the feed content.
this is how i do it if needed .
basically create the new thumbnail with the same name that the one i want to replace , upload and overwrite the existing one on my hosting server.
Building a TV Channel can be easy … if you have a right tools ===>> https://tv-channel.info/