Adding SSR to an existing finished project

[JTinkers]

I've skimmed through internet and all guides seem to be awfully convoluted.

I've tried to setup SSR on an existing project, as rewriting it for Nuxt or Vite would be an impossible feat - but I keep running into random issues like:

(node:10236) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
E:\Git Repositories\oc\app\server\server.js:1
import express from 'express';
^^^^^^

SyntaxError: Cannot use import statement outside a module

Is there a way to make it in a relatively sane way? Perhaps setup Nginx as reverse proxy and have that as SSR handler - although what about hydration in that case?

I appreciate any input