When you create an instance of FormData you need to append data to the formData object. I have seen your issue on Stack overflow.
Use this solution

const data = new FormData()
data.append("username", username)

// Go ahead and append all your variables

On your Nodejs server, if you're not sending a file along with your form, do not send form data content-type to the server. Use json instead

The reason you get the error

Uncaught TypeError: Failed to construct 'FormData': parameter 1 is not of type 'HTMLFormElement'.

is because the parameter it expects is a form element.

I can see you have const form = useRef(null);, but I can't see anywhere that you actually set that reference to your form. You need to use forwardRef in your WebForm component to target the form.

This way you can them put a ref on your component to gain access to the form element nested within it. Assuming you've forwarded the ref, you can then write <WebForm ref={form} ... /> which should then ensure your submit function gets all the form data when submitting to your endpoint.