I put my Windows 10 OS, Sonar and all my other apps, and my VST plugins on my C: drive (SSD). Sonar launches in a few seconds, and so do many projects without large sample libraries attached to the plugins.
I put my Cakewalk Content path, Command Center installer path, and VST sample libraries on my D: drive (HDD). Many VST installers will offer a choice of separate paths for installing the plugin itself and the sample content. I think my next upgrade will be to get a big fat SSD for my D: drive.
Not sure why you want to keep your old Windows install around on an internal hard drive, unless you intend to dual boot. I wouldn't attempt to share anything but document folders between two different Windows OS.
What I usually do when I migrate to a new system or OS is to make a complete backup of the old system onto an external drive. Then use that to restore any documents, projects, pictures, emails, bookmarks, etc. that I want on the new system. I keep the archived backup around in case I ever discover that I forgot to copy something, so I always have it.
But as far as applications go, I always install them clean and fresh, on the new system. Look at it this way, you might have to re-customize some config settings, but at least you aren't bringing any old bugs along on the trip!
I think it sounds like you are well on your way to success. Good luck with the build!