Only then : when/ after [you do..../ it does...]
This expression is used for somewhat big or serious matters. So often Koreans use this for a trivial stuff as a joking manner. (For example, give me a kiss. 그래야 비로소 I will leave you alone. ^ ^)
I will raise my children so they can become independent and be on their own. 그래야 비로소 Only then [=after all that I had said in the previous sentence was fulfilled], I could leave.
Please note this is somewhat written expression more than colloquial. Very commonly used in a literature or in a newspaper article.
English 'Only then' --> 'then' focuses on 'time'
그래야 비로소 ---> the focus is on '그래야' and it emphasize the precondition of the following sentence =
only after the previous condition was fulfilled, right there and then (the following sentence will be true.)
I hope this helped! Happy learning! ^ ^