We were in a similar situation to you last year in that we wanted to book for November 2019 but the Free Dining deal was not available until April of this year when the booking window opened.
I'm not going to lie, trying to get onto the site & get things booked on the day that the booking window opened with the free dining deals was a nightmare but we got there in the end, & it was definitely worth waiting to get the deal.
We've looked at the figures & are convinced that the free dining deal makes more sense financially for us than the free nights one, I'm not saying that it would be the same for everyone but it works for us.
If free dining is your thing then I would recommend holding off booking until next year, I've just checked & we booked on the 20th of April this year.