How can I produce a non-blocking process to modify newly created nodes of custom content type?
As an alternative to the cron job, this sounds like the queue system might be better.

The main difference is that in your hook_cron(), you have to execute queries to find data to process. With a queue, you listen to new entities being saved (hook_entity_insert()) and put them in your queue.

Then Drupal will manage the list of tasks (queue items) and will automatically invoke your callback for each of them. It will also handle time management and process the configured amount of queue items until the time you configure on your queue is up.

There are plenty of examples and blog posts to be found, this one seems pretty good: https://www.sitepoint.com/drupal-8-queue-api-powerful-manual-and-cron-queueing/. It seems to be very close to your use case, with a hook_entity_insert() than then does stuff with the new entities. It's pretty complete and does some more advanced things like a form where you can check the pending items and fire them off manually. You can of course skip that if you don't need it.