"sonda" is not correct Turkish, at least which is spoken in Turkey. but among other Turkic Languages one may use like "sonda".
actually an "invisible possessor" exists. it is something like "bunun sonunda" which "bunun " is usually omitted unless you want to mention a specific event..
"bütün bu olanların sonunda"= "at the end of all these happenings"
just a little note to add. When you use "sonunda" in the meaning of "at the end" you use with an object like "konserin sonunda"=at the end of concent or "caddenin sonunda" at the end of the street" so sonunda is like "at the end of.." If you use "sonda" for "at the end" alone It may be better since when you say "son-un-da" the "un" in the word makes it belong to something, "end of something"
If I was in a taxi (for example) ans someone asked me where in the street, my house was, I'd say "at the end". If I was in a concert, and someone said, when do you sing your song, I'd say " at the end". Both of these could not be replaced with finaly. In fact, the only time Finally would be used on its own would be if you were expressing emtion or sarcasam. I've been waiting for you to arrive. You're 30 minutes late, and when you turn up, I might say "Finally".