I should have had some patience.. instead of posting the question here first. I think I have a solution.

~/bin/clean_old_projects.js

const { readdirSync, statSync, rmSync } = require('node:fs')
const {execSync} = require('node:child_process')

const getDirectories = source => {  
  try {
    return readdirSync(source, { withFileTypes: true })
      .filter(dirent => dirent.isDirectory())
      .map(dirent => `${source}/${dirent.name}`)
  }
  catch (e) { return [] }
}

targets = [process.env.HOME]
nodeProjects = []
for (let target of targets) {
  if (/node_modules$/.test(target)) {
    nodeProjects.push(target.replace('/node_modules', ''))
  } else {
    targets.push(...getDirectories(target))
  }
}

const sh = fh => `find "${fh}" -name node_modules -prune -o -type f -print0 | xargs -0 stat -f "%m %N" | sort -rn | head -1 | cut -f2- -d" "`

const fourWeeks = 2419200
const now = new Date()
for (let target of nodeProjects) {
  const fh = execSync(sh(target)).toString().trim()
  if(now - statSync(fh.length? fh: target).mtime > fourWeeks)
    rmSync(target + '/node_modules',  { recursive: true, force: true })
}

then crontab -e and save:

0 3 14,28 * * /usr/local/bin/node /Users/MY_HOME_DIRECTORY/bin/clean_old_projects.js