Many have employed the same strategy you are considering, buy at SSR and then use the 7-month window to reserve elsewhere. You mention staying mainly at Poly. Based on currently existing reservation patterns, that would mostly work for the DVC low to moderate demand season at WDW that runs from the Monday after marathon weekend in Jan to late Sep. It is a more questionable strategy for the high to extremely high demand season that runs from late Sep to marathon weekend in Jan, during which time Poly studios are often booked full before 7 months out (and during that season, many things at WDW can disappear before 7 months out -- even SSR rooms can disappear once in a while before 7 months out during that season). Also, if seeking 1BRs at other resorts, you can often even have a chance at other resorts during that high demand season.
That strategy also comes with two additional caveats. First, reservation patterns can change and there is always some risk your desired 7-month reservation at Poly could become difficult to get much of the year. Second, there is an issue not mentioned above because it has never happened before, but the "new" DVC that exists today, which management seems to relish going out of its way to make things far more difficult for existing members, including particularly for any resale purchasers, may consider doing it.
If you look at the existing
point charts you will see that the same point requirements apply to both the 11-month and 7-month reservation windows. That has always been the practice, but unbeknownst to many, the POS documents specifically allow DVC to change the 7-month point charts to be entirely different from the 11-month point charts and, unlike for the 11-month point charts, there is no rule for the 7-month point charts that would require any increase in nightly points for rooms to be met by a like decrease at other times. In other words, to lower demand at 7-months out for the near park resorts (and simultaneously have a DVC desired negative impact on the resale market), DVC could create 7-month point charts that greatly increase the points needed to reserve the near park resorts at 7 months out.