There are really small things that I always have to do like telling BrowserSync that it always should open Chrome, but also larger things like installing custom gulp apps that I like to use often.
How would you go about (automatically) adding these to sage every time I start a new project.
You could fork the repository and make your changes there 
I have my own “fork” of Sage (A mix of Version 9 and 8.5 in my case) and composer works in the same way!
Why would you not be able to use Composer?
I’m assuming you meant you cannot create the project from composer, such as running composer create-project roots/sage your-theme-name dev-master. In that case, yes, although you can publish your own repo on packagist and do the same but with your project name instead of roots/sage