This can be done with a single Dropbox action step. I suggest you start with the “Append to Dropbox Journal” action you mention and duplicate it (tap and hold on action in list of options).
Edit the new action, drill down to the Dropbox step, and make changes to the field values to match what you describe above:
- Change file name and path to target the file you want to use. You do not need to create the file, Drafts will if it doesn’t already exist when you run the action.
- Change the “Write Type” to “Prepend” (beginning of file) instead of “Append”.
- Edit the template to include your title and body, something like:
## [[title]]
[[body]]
You can learn more about using templates in this article.
Hope this helps!