Use a variable to allow the player to perform an action if the button is held. Place the variable in a Coroutine to allow the user to repeat it over. isFiring in this case, it stops the Coroutine when set to false.

firing.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.InputSystem;

public class Firing : MonoBehaviour
{
    [Header("Weapons")]
    public GameObject mainWeapon;
    public GameObject backupWeapon;

    private GameObject Player;

    private PlayerControls playerControls;

    private PlayerInput playerInput;

    private bool isFiring;

    // Start is called before the first frame update
    void Awake()
    {
        Player = this.gameObject;
        playerControls = new PlayerControls();
        playerInput = GetComponent<PlayerInput>();
    }

    private void OnEnable()
    {
        playerControls.Enable();
        playerControls.Player.FireMain.performed += FireMain;
        playerControls.Player.FireMain.canceled += FireMain;
        playerControls.Player.FireBackup.performed += FireBackup;
        playerControls.Player.FireBackup.canceled += FireBackup;
    }

    private void OnDisable()
    {
        playerControls.Disable();
        playerControls.Player.FireMain.performed -= FireMain;
        playerControls.Player.FireMain.canceled -= FireMain;
        playerControls.Player.FireBackup.performed -= FireBackup;
        playerControls.Player.FireBackup.canceled -= FireBackup;
    }

    private void FireMain(InputAction.CallbackContext obj)
    {
        // Button is held down
        if ( obj.performed )
        {
            isFiring = true;
        }
        // Button is released
        else
        {
            isFiring = false;
        }
        // Run the Coroutine
        StartCoroutine( loopFireMain() );
    }

    // The Coroutine to run
    IEnumerator loopFireMain()
    {
        // Check the button is held down, do nothing if it's not
        if ( isFiring )
        {
            Ray mainRay = new Ray(Player.transform.position, Player.transform.forward);

            if ( Physics.Raycast(mainRay, out RaycastHit mainHit, 100f) )
            {
                // hit code here
            }
            yield return new WaitForSeconds(0.2f);
            StartCoroutine( loopFireMain() );
        } 
    }

    private void FireBackup(InputAction.CallbackContext obj)
    {
        Ray backupRay = new Ray(Player.transform.position, Player.transform.forward);

        if (Physics.Raycast(backupRay, out RaycastHit backupHit, 100f))
        {
            // hit code here
        }
    }
}