Adding readmore tag to webhook to joomla content article

Hello, i set up a webhook for fetching automatic generated articles from a third system. This webhook is connected with a joomla app - to create a article inside this cms system.

The situation is, that the content is one block - as i manually have to add a readmore to the article, so that the user in blog view can see only the short intro-text and then the readmor-button.

So my question is, is there a solution / idea / … to set up, that a readmore tag will be added automatically to the content — for example after 500 characters, or 3 blocks … something to get a better structure?

Thank you for your assistance.
Best
Frank