I think the approach here is to code these basic features by hand. What's preventing the author from building what they need on top of their basic stack?

Speaking as someone who hand-rolled their server stack as a C++ framework for years, there is nothing in there that can't be done relatively easily. Some things certainly get easier even, the actual change in paradigm here is not that it's a hand written web app server in C, it's that it runs in its own server container which you have to manage. That's the actual tradeoff. You can't just deploy to Vercel.