I wanted to Separate
Shipping methods from Shipping Step in Magento 2 on Checkout page ?
I propose you two solutions :
First solution :
You can create a new step in checkout like in the documentation : [https://devdocs.magento.com/guides/v2.3/howdoi/checkout/checkout_new_step.html]
In this step, you create a new form with login/password fields and send data with a custom webapi.xml to your custom controller to process login like Magento.
Second solution :
You can create a new page with controller and layout (classic process). In you controller's method, you check if the customer is loggin or not. If he is, you redirect your customer to checkout.
Create a controller to receive your new login form and redirect the customer to checkout after execute the Magento login process.
Don't forget to edit checkout urls in cart pages.
For best practices, I suggest you the first solution, but if you don't want your login page in checkout or if you don't want use Knockout, you have the second solution ;-)