I was there from November to march inn 2007-2008 and there were good waves the whole time minus the first 3 weeks of November which are normally good. I stayed around pichilemu the whole time and whenever I went north or south the swells just did not get in at the same size. The best month was January and all the locals say the off season is better as the swells come in at a more west direction. The biggest days were 15 to 20 feet in January but I am pretty sure that is very rare. All in all its never a bad time in Chile all year, it just picks up so much swell. I mainly went at that time because the kiting was better but I surfed way more than I kited. Your best bet is to stay close to pichilemu though, it catches way more swell.
With the exception of the stupid covid lockdown years, I've been going down there during the holidays to visit family every year for the past 15 years. Heading down again next month. Although my wife is from the 6th region, we prefer to drive three hours south to the zone that you mentioned. The sand at a few of the points isn't the greatest during this time of year, but there are a lot of options. Although it's smaller than the rest of the year, there's still plenty of SW swell filling in. There are lots of super surazo south windy days in December/January. The Northern Pacific W swells can actually be pretty solid as well. December is the most uncrowded time of the summer. The area you mentioned was pretty empty last year. The hoards of city dwellers come after the new year for their yearly beach vacation.