I have dealt with the exact scenario you are describing. You have a couple options, neither of which are likely ideal.
The easiest (by far) would be to buy more Flex SOs. Even if you buy resale, they should work at any of the Flex home resorts just like if purchased from the developer, you just won't be able to use them in other resorts in VSN. But with Flex, you have a pretty good choice of home resorts where even resale maintains its "FLEXibility." There are two downsides: (1) There are supposedly no Oceanfront units in Flex. So this means at best you will have an "Ocean View." Since they assign rooms based upon confirmation date, if you book right at 12 months out, you will probably have a pretty good location. (2) Flex lets you book 1 to 14 nights at once. So you will have to make 2 or 3 reservations to get your month. But since there is quite a bit of Flex inventory, if you do this right at 12 months out you should not have an issue getting a reservation.
If you want an oceanfront unit, your best bet it to buy deeded oceanfront weeks at either of the WKORV or WKORVN. While these units are some of the more expensive on the resale market, the resale price is still way lower than an original developer purchase. These will allow you to book an oceanfront unit 12 months out, 1 week (and always a full week) at a time. So for a month, you would have to get on and do this 4 weeks in a row, and in popular times these units fill very fast. So there is a risk of not getting a unit at the time you want. Also note that at WKORV there are two types of oceanfront units, deluxe (on the corners) and the center units. The two types are separately deeded and you can only book the type you own at 12 months out. WKORVN has more oceanfront units, but many of the views are not as good. Both are also "mandatory" resorts, so you will also have SOs that can be used in the network at 8 months, just like an original developer purchase.
I actually have both options. I have the deeded oceanfront weeks that I use during the last half of April and early May (mud season in Colorado). That is a low demand period, so it is easy to get the reservations.