It simply wasn't quite what I was looking for. The popup was a bit annoying and in the way (and I didn't want it to show up just for highlighting text) and the context menu was inconvenient since you had to go through two menus to use it. I was looking for something that added a quick-search option to the right-click context menu as far up as possible. I can say that the plugin looks well designed, though, but it just didn't have the features I wanted :)
UPDATE: I see, thanks for explaining! I'm not sure if you'll get a notification for this reply, but oh well.
For the popup being annoying there are options to make it appear only when you click a key combination, or move it away from the cursor, for example. The options page has many of these. :)
As for the context menu being a sub menu, unfortunately that's, as far as I know, a WebExtensions limitation that I can't do anything about :( (Unless there's a single search engine!) See this for more details: https://github.com/CanisLupus/swift-selection-search/issues/129