I read the first time around that it’s on v8.5.1. Doesn’t change the fact that with either version of Sage it’s easy to add separate files to be minified. On 8.5.1 that would just be a new section in the manifest.json, like I said in my first response. The Sage book covers all of that btw.
You could do it almost as simply as you do it with just normal Bootstrap if you’d like to do it that way. There are multiple ways to go about this, with varying degrees of how much WordPress or Sage would even be involved. If you’re going to use the manifest to create a separate CSS file, you’ll want to make sure to enqueue it in the setup.php file along with the main stylesheet.
I’m sorry in that I’m not going to be able to provide a step by step simply because I’ve got to get my own work done. We’re available for hire if you need personalized help with coming up with a solution for it as well.