> Much like there wasn't one "Caesar", since it's a title
It wasn't (yet) a title during the lifetime of Gaius Julius Caesar, though...
> still kicking in Proto-West Germanic derivatives branching off "kaisar"
Like the non-Germanic Slavic "Tsar".
> Sextus Julius Caesar in around 200 BC, 300 years before Gaius Julius Caesar was born.
Did you accidentally swap the 200 and 300, or are you saying Julius was born ~100 AD?