The essence is that Atmel START is a framework for many MCUs and it is relatively easy to use.
However I don’t think it can be made as a library as it generates the vector tables for the ARM mcus, and some overrides of stdio in case it is requested.
My question would be what is the way to bring the Atmel START to PlatformIO?
How much effort do you think it will be?
From what I see it will be some work to generate the SCons build files instead of CMake, and the work to add the framework to PlatformIO, which I can not estimate.