Create a custom self-registration page and Apex handler that creates a person account for each user self-registering on the portal.

Use the standard self registration configuration under Experience Workspace and leave the default Account field empty.

Use the standard self-registration configuration under Experience Workspace that assigns users to a default business account, and then build a trigger on Account to create person accounts for each user.

A Restrict the Account record type access to Person Account record type only for the Site Guest User, so any account created is By default a person account.