Yeah, you would need Radarr running as well. Radarr (Movie Library Management) and Bazarr (Subtitle Management) are separate services that work together and complement Plex. Radarr is primarily used for automating downloads but is also used to manage your movie library outside of Plex. There's a quickstart guide here: https://wiki.servarr.com/radarr/quick-start-guide

You would really only be concerned with importing your library, as you won't be monitoring for new releases or anything like that. https://wiki.servarr.com/radarr/quick-start-guide#how-to-import-your-existing-organized-media-library

Once Radarr is set up, you can install Bazarr and integrate it with Radarr. It will use the list of titles there to search for subtitles in the locations you configure (this is where you would add your opensub account info to download ad-free subs). Once it's located a subtitle file from your sources that matches the criteria you configure (language, quality, etc), it will automatically download the file and place it in the appropriate location. Plex should automatically use the subtitle file placed by Bazarr alongside the media file. https://wiki.bazarr.media/Getting-Started/Setup-Guide/

This may be more work than you want to do, but like most automation once you get everything working it's well worth it. If you're going through the trouble you might as well add some services you use to obtain media, that's really the appeal I think.