For permission in a SharePoint list, here are some information which may be helpful to you.
I assume you are clear about how to configure external sharing in Microsoft 365 to grant access to patients. And here is an overview if you want to have a check on it.
We can share a list with a customized Power Apps form to these external users. Make the patients to fill in the form with the PHI. Through setting the item-level permission , we can ensure the external users can only see the forms created by themselves.
For the appointment with teams, it may be better to send the invitation via Outlook Emails with a link for a Teams meeting. User can join the meeting with the link. External users shall be fine for this part.
And for the auto-generated Email, you may turn to Power Automate to create a Flow to generate the Email with required information, it is more handy and very powerful. We have actions in flow to create sharing links. So things could go like this, when a required document is created, trigger a flow to generate a sharing link to that file, then send the Email to the specific external user (patient) with this link. The users only need to receive the email and access the file we provide a link. Guest access shall work.
For auditing, Microsoft 365 provide the audit log for you to monitor user activities.
So overall, it shall be fine for you to handle the whole process with a guest account except the part about “after-chat” and paperwork as I am not very clear about your detailed requirements. Still, as this procedure you describe needs a lot of different products, it will always be helpful for you to have a talk with a MS partner or solution provider.
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