Replace string with new line + string in a big file
Try

sed -i '' $'/),(/ s//),\\\n(/g' bigfile.sql



$'...' indicates that the string is a C-style string. This allows you to use the \\ and \n escapes, among others.
In place of your \n, we have a \\ and then a \n. This passes a literal backslash, and then a literal newline, to sed. This allows it to recognize that the newline is a part of the pattern, instead of terminating it.


The following also works:

sed -i '' '/),(/ s//),\
(/g' bigfile.sql