Hi, not that I know the answer but I am on the same boat 
I watched this guy âAlex the Enterprenerdâ used magic for easy passwordless Auth but I too donât want to lock into paid product here.
I too want to use nextAuth with passwordless flow and at the moment do not know how I am going to do it. I am too far away from this stage yet. But when I get to implementing authentication, I will have to dig deep into this topic.
After some brief investigations I was considering to just write a custom database adapter for nextAuth that would talk to strapi API under the hood. Basically, use strapi as a storage.
I also saw someone else approached it differently: How to use NextAuth.js with Strapi? ¡ Discussion #574 ¡ nextauthjs/next-auth ¡ GitHub
And there was a blog explaining how to hook up strapi and nextAuth User Authentication with Next.js and Strapi
But I didnât like the approach. It assumed that the nextAuth can access the DB that strapi uses.
Imagine if your frontend is hosted on Vercel or Netlify and strapi backend somewhere on your own AWS account. I would never consider exposing my internal DB to a public network directly.