[preommr]

Fastify has a nice modular architecture, decent popularity, growth trending upwards (so more future proof), first-class support of async and typescript (it's not cobbled on like it is with express).

Conversion was mostly easy because most web frameworks are pretty similar. It's less an issue of an individual frameworks features and more how popular it is. For example, a lot of people use passport for auth, so using a popular solution is more likely to have a well-supported plugin for passport, which makes conversion trivial.

On a practical note, I did have some minor roadbumps with the typical PITA issues like dealing with file-streams and multipart.