OP, I book my Fort stays as soon as I know I'm going to be going. Since we typically have been limited to going to Disney within narrow times of the year (the first week or Jan or mid-March, mid-Apr) we book about a year out minimum. Holiday times particularly Halloween and Christmas-New Years book at the max window ideally. Locals book up weekends early October-April or so which makes it hard to get week-long reservations (Sunday-Thursday sure but hard to get Friday and Saturday night during those times unless you move early).
In these days of social media and fast communications, most of us find out about special deals before they come out usually or we know that certain offers follow an annual cycle ("usually free dining offers come out in xxxx month"). Travel agents really have no special tools other than the diligence to check frequently for openings on behalf of their clients. I was amazed in talking to one TA that, for example, when trying to book for the following year before the on-line tool is available, they have to call in (just like we do) and sit on hold (just like we do) and give the Disney agent the necessary specifics (just like we do).
But many of us like to do our own frequent searching to fill holes or patch a stay together and this board is full of those past success stories. And we know how to get them patched together into a single reservation (it's been widely discussed on this board). Linking reservations ("travelling with") so that people who want to get sites assigned next to each other) has also been widely discussed here in the past. So you can do it yourself or use a TA - the choice is yours. There is a trade-off to using a TA of course so everyone's decision is their own. No one size fits all.
I'll close with a couple pointers:
1. Disney has become tougher in the last year or two about people who book a longer vacation and then come back later to "drop" some days off but "keep" the reservation. Disney tries to avoid stranded capacity that won't get booked if the time frame is too close to travel. We may think of it as a "change" to the existing reservation and the agents use that terminology but in the background it is truly a cancellation of your old reservation and booking of a new reservation (and sometimes they reuse the same reservation number). Say you have two weeks booked for Christmas and you call up and say you want to change it to just a single week. It holds your current reservation and looks for a NEW week in the background. Christmas might be booked full so it can't find one. Wait, you say. All I want is to drop a week off an existing reservation so I know there is one - MINE. Nope but keep trying till there is one.
2. Same works in reverse. You want to add two days to an existing reservation? You can see the availability of the two days in the system? Whaddya mean I can't add those two days? I see them! (Again, it's holding your current reservation and searching for a new reservation that covers those two days PLUS the length of your existing reservation). If there is a conflict with finding availability parallel to the current reservation, it will not work. That's when you book the two days as a separate reservation and later merge them into a single one (assuming consecutive dates and same level of site). I learned this lesson a while back after an angry conversation with an agent.
3. Disney changed their booking system when they loaded 2020 rates last year. The old method that you could book 500 days/499 nights in advance of your GOING HOME day (in other words, your whole reservation had to fit inside the 500 day window). With the 2020 rates, the system now looks at the 500d/499n from your ARRIVAL day (maximum of 30 day reservation). So now use your ARRIVAL day in the 500d/499n calculation for the max window.
4. If you can't get what you want, as others say, just keep trying. Like above, try booking in smaller chunks or day by day to show/get what you can and try to piece it together. You have to be diligent but the further in advance you start this approach, the more successful it is. Cancellations do occur and openings do happen for various reasons (as you found out on your last trip) so it can be successful.
Sorry to be so wordy. Use a TA if you want. I don't and it really hasn't been an issue. Good luck!
Bama Ed
