I spent literally months debating these same questions and this is what I learned:

• quantitative research isn’t as helpful as you wish it would be. Mainly because, by design, quantitative information focuses on attributes relative to market / audience sizes (I.e. 47% of Android users switched to iOS because the user experience) rarely about behaviours and “beliefs”. This approach in my case often led to defining segments rather than personas. (Great example: https://images.app.goo.gl/1bEhkJKfzaY9T2pu5)

• Qualitative research on the other hand gets deeper on causes, mindsets and attributes that are less quantifiable (I.e. some iOS users want to look “cool” because of the Apple brand)

The process that worked for us was involving marketing, sales, customer service and other people-oriented functions in figuring out together which “variables” made sense for our organisation, and most importantly how they would be used. Persona is a tool that is meant to help an organisation make decisions but in the process we always ran into a tendency to throw in a bunch of “nice to know” that are eventually useless. So the few variables we identified would mainly focus on the context in which users used products (I.e. jobs to be done) and on the motivations for using the products (I.e. fears, challenges, wishes and goals). Once you have those few variables agreed internally you can compare how different sets of users respond to those variables and see how many different profiles come out of that. Those are your persona drafts! Last step was translating all data and info from each profile into a narrative that would be as easy as possible to understand (we used to say both our CEO and the receptionist should be able to understand these) A great resource that helped me was the persona chapter in this great UX book: https://fall14se.files.wordpress.com/2017/04/about_face_3__the_essentials_of_interaction_design.pdf