Free NTLite can process images in parallel except it has these nag screens that constantly stop and ask you to Continue.
In all honesty, if you're writing a tool to customize a range of base images then you should just familiarize yourself with DISM and script applying updates and drivers. You might as well start by applying SP1 to RTM.
And there's no need for the zoo. Make your tool work on the base SKU's (Home, Premium, Pro, Ultimate, Enterprise) and one of the token N or KN images. If the logic works, then the only difference is collecting all the licensing variants.
Best of luck.