Hi there and thanks for the review! :)
I understand, but it's hard for me to maintain a decent list of search engines up to date for SSS's users, so I prefer if people add their own engines. You can also import the ones you use in the browser. People suggested creating a webpage where users could add the engines they are using, like a Wiki, though that would also have to be moderated and it's really hard to do that in my free time.
Regarding the unnecessary settings, I also don't use most of the options, but almost all were user-suggested via email, GitHub or in reviews, so someone is finding them useful. ;) If you have ideas about what to do about them without removing them, I accept feedback!