Shopify Does not allow us to customize checkout, so we may need to check for a limit on the cart page. 

so after login / Register, we need to redirect the merchant to the cart page. to do so please follow below steps.

Add Below Code in theme.liquid

<script>
  $(document).ready(function(){
    var $returnTo = $('<input type="hidden" name="return_to" value="/cart" />');
    if ($('#RegisterForm').length > 0){ //if registration form
      $returnTo.insertBefore($('#create_customer input[type="submit"]')); // add return_to input
    } else if ($('#g-recaptcha').length){ //if captcha form
       $returnTo.insertBefore($('.shopify-challenge__container input[type="submit"]'));
    }
    var cu = $("[name=checkout_url");
   
    if(typeof cu[0] != "undefined"){
      cu[0].value = "/cart";
    }  
  });
</script>