I'm facing the same problem that you are with STIG requirements on SQL databases. I've mitigated this by creating two separate audits. One includes all required STIG audit groups with the exception of 'SCHEMA_OBJECT_ACCESS_GROUP'; I've limited this to a reasonable log file size and number of files to last until my weekly backup where my logs would then be offloaded to my backup server.
The second audit I run is only for the 'SCHEMA_OBJECT_ACCESS_GROUP'. I've allocated the size and number of files I am able to support. In this way, I do have the audit enabled.. I just don't let it overwhelm my system; I do not retain more than a few hours of logs for this audit. I know its not completely meeting the intent of the STIG... but all the noise generated in that log renders its output nearly useless and in the end it is better to have a functioning server than to execute a DoS on yourself.
If you ever find a way to exclude SQL_Agent processes from being logged or some other useful way of filtering out the garbage generated, please let me know.
Thanks!