You shouldn’t need thumbgen, as it looks like it is designed to mimic what Kodi skins do: show you all the information you want to display.
All Kodi scrapers will grab posters and backgrounds (fan art), and you can also download other artwork from fanart.tv (logos, clearart, etc.). For the extra art, you’ll have to use some kind of artwork manager. There is a not very good one as a Kodi add-on (Artwork Downloader), or you can use various apps that download and organize the extra artwork.
I use Media Center Master to organize my artwork, but there are a lot more programs available.
I have about 800 movies, and it only takes about an hour to do a full scrape and download of all artwork for them. That’s a one-time thing…after that, adding a movie takes a less than a minute.