I’m not seeing any issues here? Though keep in mind that you don’t need to start writing the script from scratch - you can take the script generated from the .har conversion and start from there, deleting requests to external websites, adjusting headers, etc.
And, in general, if your login page returns the credentials as a JSON object, you can save the http.post() result in a variable and access them that way - see Response
And if the website uses cookies, then that should be handled mostly automatically by k6, though you can also manually adjust them, if necessary: Cookies