is it possible to restrict an action method to only redirect-to access?

There is an attribute Referer in the header of Request. If it is accessed from a browser, its value is empty. Use this to determine the subsequent processing procedure.

[HttpPost]
[ValidateAntiForgeryToken]
[Route("MainActionMethod")]
public async Task<IActionResult> MainActionMethod([FromBody]object jsonData)
{
   if (true) 
       return RedirectToAction("Action1");
}
public IActionResult Action1()
{
    StringValues header ;
    Request.Headers.TryGetValue("Referer",out header);
    if (header.Count==0)
    {
        return BadRequest();
    }
    return Ok("Action1");
}