Welcome to the Auth0 Community!
I am sorry about the delayed response to your inquiry!
As you have stated, a limitation of the Forms and Flows feature is the fact that it is tightly tied to the PostLogin Trigger instead of having the flexibility of using it in any desired trigger. You can submit a topic or upvote existing ones on our Feedback page in order to bring higher attention to our product team.
Otherwise, as an alternative, you should be able to use Partials in order to add these custom fields directly on the Signup page instead of redirecting to a form while also being able to handle these variables using Triggers. This will help you handle the normalization of the organization names.
Also, as a workaround for you Individual and Business users problem that you are encountering due to organizations, you could have a dummy/default organization set up on your tenant in which all new users are added after registration until they receive and accept the invite for the one created. This way, you will be able to deny access using a PostLogin trigger if they are a part of the default organization. Please keep in mind that you will need to remove these users from the default organization once they have accepted the invite.
If you have any other questions, let me know!
Kind Regards,
Nik