I’ll add in a post create with featured image template eventually to our Max templates. In your case since you are using ACF your form will be slightly different. The overall action flow will be Post Create > File Upload > Post Meta Update / ACF Post Update. Make sure File Upload has Has Multiple turned off otherwise you will have to loop over the images.
In your ACF Post Update action you are going to utilize Action Response Dynamic Content. When you use the Post Create action, your response is the new posts ID. Since it is two parents away, the depth will be 0. Put together that is {{ form_action.response({"depth":"1"}) }} in Twig. You can use that in the Post ID field. The File Uploads response will be the ID of the created media item. Since your ACF action will be a direct child of that action you can use {{ form_action.response }} in the meta value field. My screenshot shows setting that up for a Featured image.
Let us know if that helps and have a great day.