If you leave the “Path to your Login Page” option empty, the site’s default login page should get used both for regular user logins and logins that are triggered from the WP Discourse SSO code. Do you run into issues if you leave that setting empty?
Looking at the code, I’m unsure about what normal case would require the “Path to your Login Page” option to be set. This is based on the assumption that plugins that are adding a custom login page would already be hooking into the login_url filter so that the custom login page will get returned by calls to wp_login_url.
For anyone interested, the relevant code is here: https://github.com/discourse/wp-discourse/blob/main/lib/sso-provider/discourse-sso.php#L140.