On March 14, Southwest will accept bookings for flights between 10/02/2019 and 11/02 2019. I suggest looking at dates/fares as early as possible on March 14, and booking, if possible, at that time.
As Southwest does not charge a fee for changing plans, if paying with cash or points, you will have a credit for cash tix on your account for any fare reductions or cancellation. That credit must be used for travel within one year from original booking date. So, you are not risking much by booking ASAP, and might very well save over waiting. You can always keep checking to see if your flight has gone down in cost. If booking with RR points, the points and fees would be returned and fully credited for changes/cancellation.
Dates for Sept. till Oct. 1 were probably heavily booked when the last extension occurred, so many of the lower-cost seats were probably sold out awhile back. As the flights fill up, the seats will get more expensive - supply and demand at work.
In case you are not aware, Southwest has a “low fare calendar” that can give you a quick look at day to day fares, which might be useful.