Customer Details based on Shipping

How would I easiest make the required Customer Details change based on selected Shipping module? Like if a local pickup is selected there is only Name, Email and Phone inputs or only those become required, I guess I would use update() to trigger something in box_checkout_customer.inc?

There are two validations taking place.

One is for the customer details before storing them.
https://github.com/litecart/litecart/blob/dev/public_html/pages/ajax/checkout_customer.inc.php#L34-L54

The other is before we are creating an order.
https://github.com/litecart/litecart/blob/dev/public_html/includes/entities/ent_order.inc.php#L569-L644

During the first part we have a "hen and the egg" dilemma. We are collecting customer details for passing them to shipping and payment. Meaning you don't really have a choice of shipping to rely on at this early stage.

So what you are saying is its not really possible, maybe another approach, selecting a shipping module say local pickup, or one which doesnt require as much customer details just fills the customer details with "-" ?

I think you can remove all validations in checkout_customer, then use a condition for ent_order that checks if ($this->data['shipping_option']['id'] == 'sm_module:option').

possible solution:
file: includes/templates/default.catalog/views/box_checkout_shipping.inc.php
At the very end add this js:

<script>
$("label.option").on('click', function(e) { // this is the label with class option
  e.preventDefault(); 
  var url = '?shipping-method=' + $(this).find('input').val(); // add string to url like: ?shipping-method=sm_pickup:pickup
  window.location = url;        
});
</script>

file: includes/templates/default.catalog/pages/checkout.inc.php
Add this at the very top of the file:

<?php 
if( !empty($_GET['shipping-method']) && $_GET['shipping-method'] == 'sm_pickup:pickup') { // read $_GET from url; local pickup is chosen
  $_SESSION["local-pickup"] = true;  // create a session
}
else {
  unset($_SESSION['local-pickup']); // if not sm_pickup:pickup in url; unset the session for local-pickup
}  
?>

Now the client side validation:
file: includes/templates/default.catalog/views/box_checkout_customer.inc.php
Check the fields which are required and you dont want them to be required when local pickup
e.g.

      <div class="row">
        <div class="form-group col-sm-6">
          <label><?php echo language::translate('title_address1', 'Address 1'); ?></label>

          <?php 
      if(!isset($_SESSION["local-pickup"])) {
        echo functions::form_draw_text_field('address1', true, 'required'); 
      }
      else {
        echo functions::form_draw_text_field('address1', true);   // delete required
      }
      ?>
        </div>

Finally the server-side validation:
File: includes/entities/ent_order.inc.php
Look for

public function validate($shipping = null, $payment = null) {

Below add the session check

    public function validate($shipping = null, $payment = null) {

  // check if is a local-pickup
   if(isset($_SESSION["local-pickup"])) {
     $localPickup = true;
   }
   else {
     $localPickup = false;
   }

and later in

    // Validate customer details
      try {
      ...

you can bypass the validation:

      try {

        if (empty($this->data['customer']['firstname'])) throw new Exception(language::translate('error_missing_firstname', 'You must enter a first name.'));
        if (empty($this->data['customer']['lastname'])) throw new Exception(language::translate('error_missing_lastname', 'You must enter a last name.'));
        if (!$localPickup && empty($this->data['customer']['address1'])) throw new Exception(language::translate('error_missing_address1', 'You must enter an address.'));
        if (!$localPickup && empty($this->data['customer']['city'])) throw new Exception(language::translate('error_missing_city', 'You must enter a city.'));
       ...

Thanks for detailed instructions, that could be a possible solution I may try next, however something with less change
I added did this in
https://github.com/litecart/litecart/blob/dev/public_html/includes/templates/default.catalog/pages/checkout.inc.php#L363

// Check if the selected shipping option is sm_pickup
    var isPickup = $(this).find('input[name="shipping[option_id]"]').val() === 'sm_pickup:pickup';
    var fields = {
        firstname: $('input[name="firstname"]'),
        lastname: $('input[name="lastname"]'),
        address1: $('input[name="address1"]'),
        city: $('input[name="city"]'),
        postcode: $('input[name="postcode"]')
    };

    var isPickupMode = !isPickup && (fields.firstname.val() === '-' || fields.lastname.val() === '-' || fields.address1.val() === '-' || fields.city.val() === '-');
    $.each(fields, function(key, field) {
        if (!field.length) return;
        field.prop('disabled', false);
        var val = field.val();
        if (isPickup && !val) {
            field.val(key === 'postcode' ? '10000' : '-');
        } else if (!isPickup && (val === '-' || (key === 'postcode' && val === '10000' && isPickupMode))) {
            field.val('');
        }
    });

    var customerData = ($('#box-checkout-customer').length ? $('#box-checkout-customer :input') : $('input[name="firstname"]').closest('form :input')).serialize();
    queueUpdateTask('customer', customerData, true);

Seems to work fine, keeps real data if entered and if pickup option and no data adds "-" to the fields and "10000" to postcode to pass validation and leaves email and phone, any issues I am missing here?

Yes this also works fine. Personally, it's not my favorite to place hyphens and postal code 10000 to bypass the validation.
I shortened the code a bit:
File: includes/templates/default.catalog/pages/checkout.inc.php

  $('#box-checkout .shipping.wrapper').on('click', '.option:not(.active):not(.disabled)', function(){

    $('#box-checkout-shipping .option').removeClass('active');
    $(this).find('input[name="shipping[option_id]"]').prop('checked', true).trigger('change');
    $(this).addClass('active');

    $('#box-checkout-shipping .option.active .fields :input').prop('disabled', false);
    $('#box-checkout-shipping .option:not(.active) .fields :input').prop('disabled', true);

  /************************ADDED*******************************************/
  var selectedOption = $(this).find('input[name="shipping[option_id]"]').val();
    $.ajax({
      url: '<?php echo document::ilink('ajax/checkout_customer'); ?>',
      type: 'post',
      cache: true,
      async: true,
    data: {
      selectedOption: selectedOption
      },
      dataType: 'json',
      success: function(data) {
      if(data.required == 0) {
        $('input[name="address1"]').removeAttr('required'); // remove required 
      }
      else {
        $('input[name="address1"]').prop('required',true); // add required
      }
      }
  });
  /******************************************************************/

    var data = $('#box-checkout-shipping .option.active :input').serialize();
    queueUpdateTask('shipping', data, false);
    queueUpdateTask('payment', true, true);
    queueUpdateTask('summary', null, true);
  });

File: pages/ajax/checkoutcustomer.inc.php

/************ADDED**************/
  // check shipping option = local pickup 
  if(isset($_POST['selectedOption'])) {
    $result = [];
    $selectedOption = $_POST['selectedOption'];
    if($selectedOption == 'sm_pickup:pickup') {
      $_SESSION["local-pickup"] = true;
      $result['required'] = 0; // send back for skipping required from inputs
    }
    else {
      unset($_SESSION['local-pickup']);
      $result['required'] = 1;
    }

    echo json_encode($result);
    exit;
  }
  /*****************************/

  if (!empty($_POST['customer_details'])) {
  ...

File: includes/entities/entorder.inc.php

   // Validate customer details
      try {

        if (empty($this->data['customer']['firstname'])) throw new Exception(language::translate('error_missing_firstname', 'You must enter a first name.'));
        if (empty($this->data['customer']['lastname'])) throw new Exception(language::translate('error_missing_lastname', 'You must enter a last name.'));
        // bypass address1 and city if local pickup
        if (!isset($_SESSION["local-pickup"]) && empty($this->data['customer']['address1'])) throw new Exception(language::translate('error_missing_address1', 'You must enter an address.'));
        if (!isset($_SESSION["local-pickup"]) && empty($this->data['customer']['city'])) throw new Exception(language::translate('error_missing_city', 'You must enter a city.'));
        ...

Thanks, I tried your solution and it works as expected, will save it as a backup.
I will keep the simpler one for now as I actually want to insert the postcode and city as the "Most Popular Cities" stats still works this way, and it gives some visual to the user that the fields are clearly not required vs just empty fields.