The easiest way to do this is to change code in login controller.
/storefront/controller/pages/account/login.php
Add below line right after: public function main(){
$this->redirect($this->html->getSecureURL('checkout/guest_step_1'));
Essentially, what are you doing, is forcing any login attempt to go to guest checkout.
If you need to do this in extension, you can hook to "_InitData" of this controller and do same redirect.
It is easy and straight forward.