BREAK_ONLY_BEFORE and BREAK_ONLY_AFTER are settings for when you have SHOULD_LINEMERGE set (which you should not have set unless there is really no other way).
To cut data from the event before indexing you'd normally use SEDCMD but there is one caveat - you must of course write proper regex. And with json it's kinda hard. You can't simply get anything from one brace to another because:
1) You can have nested entities. But even if you can assume that you won't have any "substructures" in your json,
2) A closing brace might be simply contained within a string.
You can use ingest-time eval to extract a given path from a json structure using json_extract() but that's "heavier" solution than simple regex-based operations.
So it's not that easy. The right approach will depend on the actual data you have and how much you can safely assume about its structure.