I had to take a different but similarly brute force approach 
I used npm-check-updates to just bulk update every package to the latest version. And that worked for me after a mildly tedious unexpected Next 13 upgrade.
I think that the issue lies in some mismatch of Netlify, NextJS, and NextAuth versions since there are so many breaking changed at play between Next 13’s release, Netlify’s support for it, and the major EOL for the old runtime on Netlify, but 
Glad you were able to solve, cheers!