Well, that's what I say, Fennec is your only option. For now, there are no Chromium-based browsers on F-Droid (except the WebView-based browsers, which ironically, runs on proprietary software as well in the background). If you find one truly open-source and reproducible Chromium browser, I'm intrigued, but as far as I know, they all pretend to be on F-Droid (= fake external F-Droid repo) but are not done by F-Droid team (which is sad). The closest I found was https://gitlab.com/thermatk/Unobtainium but still depends on proprietary code as far as I know.