A little late the party (I was on vacation last week) but yes, I agree with @Leandro_Zubrezki. The key/value pairs approach makes sense when the fields are dynamic, but if they are fixed then I think optional arguments makes more sense.
I also agree in general that it is best to start with a mega-action, but I think it may make sense to add some field-specific actions for the most common use cases. For example, a Todoist Pack may want to have a MarkComplete action in addition to a UpdateTask action, since checking off tasks is likely the most common thing someone wants to do.
Finally, I think the upcoming addition of two-way sync will further reduce the need for field-specific actions. Directly editing the value in the sync table will serve users that aren’t as comfortable with complex formulas, and the mega-action can be used by power-users in their sophisticated workflows.