Option to delete after X days?

There is a filter that you can use for that: simple_history/db_purge_days_interval.

So if you would like to remove items older than 7 days you would do it like this:

// Clear items that are older than a 7 days (i.e. keep only the most recent 7 days in the log)
add_filter( "simple_history/db_purge_days_interval", function( $days ) {

	$days = 7;

	return $days;

} );

Yes, functions.php should do it.