Generating email in SFDC/Marketing cloud but sending it out to another system, not the recipient
You can grab rendered HTML content from SFMC, but in order to render it, the information needs to be inside the platform. Which I believe is something you do not want.
Sample call:
Host: https://www.exacttargetapis.com
POST /guide/v1/emails/[emailID here]/dataExtension/key:[DE External Key goes here]/contacts/key:[Subscriber Key that you want to preview here]/preview?kind=html
Content-Type: application/json
Authorization: Bearer YOUR_ACCESS_TOKEN
Another option if PII being stored and accessed in a 3rd party system is not an option is to obfuscate the data via a service like Field Level Encryption or similar encryption on the data inside the system.
If the data can in no way be stored in the system, but the email can be sent out - I would look at utilizing an External Content Area to call to a hosted service on your other system that will create the Health Care PII inside that service and render it at send time. This would make sure the data and processing of it remain in your secure area and not in SFMC.
Another option is to have each email be a Triggered Send Definition. You would have your secure service make an API call to the TSD endpoint with the data and external key of the Trigger you are looking for. You would then want to make sure that you do not include any of the fields in the data passed in the API call that are PII so the system will not store it.
For example:
If you pass a payload like: {Subscriberkey: "[email protected]", EmailAddress: "[email protected]", FirstName: "Gor", Medicine: "tegretol 200mg"}
but only have FirstName inside the DE (automatically includes SubscriberKey and EmailAddress), the Medicine data in the payload will not be stored in the system. It will only be used for rendering the email and then disappear into the nether.
Outside of those, I honestly don't think SFMC is your solution. I would look at email creation services to build you emails and store them in your app and have it operate 100% externally from your SFMC marketing campaigns. Or you can include some markup languages (like MJML, Inky, etc.) inside your service to create the emails.
I honestly would say that I would highly recommend sending the emails from SFMC if possible instead of through your other service. By sending from another service, you run risk on deliverability (as all the SPF, DMARC, etc that you get with SFMC will not apply in your app) and with the suffer in deliverability, you could actually also hurt your SFMC domain reputation.