As others have said - it is too late to buy DVC now to use your points in the fall, and it is also not a good idea to rush a DVC purchase with a specific booking in mind. To give you a sense of how quickly bookings go:
We bought BLT from March - April 2017. We already had short stays in August 2017 (VGF, BCV) that had been booked on rented points back in early October 2016 (10 months in advance). If we had waited to use our BLT points, 3.5 months in advance, we would not have been able to stay at VGF or BCV. At the time we closed, I booked a President's Week 2018 trip with those points - almost 10 months in advance. President's week is not nearly as hard to get as fall frenzy, and at 10 months in advance, ALL of the standards and TPV of any size were gone, and even some days in LV studios. I ended up waitlisting and getting what I wanted, but that was a long and difficult process over the next several months, and was completed about 5.5 months in advance.
We bought additional points at VGF (November 2017-Jan 2018, points in account in January) with no intention if using the points until April 2019 at the earliest. That left us the flexibility and ability to get the exact # of points we wanted, at the best price, and in the UY we wanted, without worrying about planning our next vacation.
This sort of gets back to when you said you'd be able to plan "roughly" when your trips will fall. If you're planning on going mostly in the fall because DVC resorts cost the fewest points then, well, guess what? Thousands of other DVC owners have the same plan. So for fall trips, you really do need to be able to make a reservation 11 months in advance and accept that you may have very little flexibility in changing those dates. If you're not comfortable doing that, then DVC may not be a good fit for you. (Note, this changes only slightly even if you plan travel at less popular times, like late August before F&W starts in earnest; then you may have some flexibility. For example, the last week of August you can still stay for a week in a 2BR at BRV, OKW, SSR, and AKV, but not at any of the other resorts.)