I am not sure if this answers your question but the Spark auth user is still the auth user as in any Laravel app. You can make your own routing and controllers as you would normally. The only thing Spark would be used for in your situation is the subscription and subscription management. You can build whatever you want on top of Spark.
Spark is Laravel 5.3 with Cashier and Spark. Spark is built on top of Cashier. For instance, I have a site that is a service. I have a regular Laravel site with Spark serving as the subscription manager. The only intermixing of code that I do it for Auth::user() and Spark events as well as information stored in the DB.
In your case, If you decide to use it, I would do the install...setup Spark and Stripe...then build your regular site around it (within the regular Laravel app that contains Spark). Hope that answers some questions.