dconf-editor did not work for me, but thanks to one of your links (the first
one), I was able to change the behaviour with
gsettings set org.gnome.Epiphany keyword-search-url
https://duckduckgo.com/?q=%s
in the terminal.
I could not do the same for startpage, since I could not figure out how the
url should look like with startpage (the url during a startpage search
remains the same all the time for me).
But this is no big problem, because a simple !s does the job with duckduckgo
as well.
Many thanks!
