REST API Projects/Epics automation problems

I am building an automation to automatically upload single or multiple projects of any particular type, i.e. Classic/Next-Gen and/or Kanban/Scrum (4 combinations) to Jira Cloud, together with its Epics (No Tasks nor other issues at this stage).
I am using Jira API v2. For Classic Project only need to upload the Name and Original Estimates for the Epics using the Timetracking structure in the API calls body.

So far, I can use Timetracking structure ONLY in specific Classic projects that I have manually updated their screen-schema, but I want to be able to create the Epics without manual configuration of the screen schema per project.
I tried to change the Default screen-schema but it did not work.

When I don’t change the project schema for Issue creation I get the following error:

  • "timetracking": "Field ‘timetracking’ cannot be set. It is not on the appropriate screen, or unknown."

Also, in NewGen projects I cannot use the StartDate field, getting same error even if the StartDate field is already on Issue creation window:

  • "startDate": "Field ‘startDate’ cannot be set. It is not on the appropriate screen, or unknown."
  • ** I’m able to create an Epin for Next-Gen in case I use only duedate.**

What is going on with these fields in API?
I was expecting it to be easy/trivial to create objects like these using the API.