If you have the search bar hidden, the only way to manage the search engines is ☰ → Preferences → Other → Search Engines Manager or unhiding the search bar, both of which are many clicks away. My suggestion is to add a Manage Search Engines entry to the Tools menu below Configure Toolbar. (It could even show up only if the search bar is hidden, if that's easy to do.)
If you have the search bar hidden, the only way to manage the search engines is ☰ → Preferences → Other → Search Engines Manager or unhiding the search bar, both of which are many clicks away. My suggestion is to add a Manage Search Engines entry to the Tools menu below Configure Toolbar. (It could even show up only if the search bar is hidden, if that's easy to do.)