You'll probably want to look at tools that are doing metadata scraping in general and covers as part of that. Try TmpGuy's JavLuv app or others that might be geared towards setting up a media server for Plex, Jellyfin, Emby etc.
This is a decent baseline if you just want to pull 800x600 covers from a known quantity like DMM. Personally I've been downloading covers manually the entire time I've been collecting in the time consuming fashion. Nothing beats the eyeball test for me when it comes to finding a good cover scan.
Let's face it, the lack of available high definition covers has always been a shortcoming in JAV. Some studios produce them but most do not. Some distribution sites have better quality covers for a subset of titles, eg. Duga, Xcream etc.
Since Google torpedoed Google Images and forced the use of the utter garbage that is Lens for reverse image search, I've found best manual search results through Yandex image search. There are browser plug-ins to assist with this.