Oh, this is really cool! I definitely wouldn't have thought to add a bookmark (and, admittedly, I glossed over the search section in the FAQ).
I was expecting to see something closer to an table of available search engines, containing an editable column describing the quick search shortcuts (so like Brave). Then, the custom quick searchers could be removed/hidden from bookmarks to prevent them from polluting the Top Hits.
Also, another thing Brave does well is modifying the nav bar to show "Search <engine>:" when performing a quick search. Of course, I already know my quick search engines, so this is more of a nice-to-have.