What is the best way to update/add paragraphs with REST on a node
https://www.drupal.org/project/entity_reference_revisions/issues/2848878 is an issue that allows to embed paragraphs on a field directly into the main resource.

We developed and use it exclusively for the use case of default_content with hal_json. It will likely not work out of the box with REST and patching/updating entities. But you're welcome to use it as a first step towards that, patches based on that are welcome.