Use case for Prospects profile creation and Experience Event stitching

We currently don't have any Prospect records in our Production sandbox. We only have clients (Client dataset and Schema, a Dataflow that ingests client data) and a Web Event DS (that collects web interactions). Our sales cycle is long, possibly up to 6 months or more from awareness to conversion.

Use case: Ingest Prospect data from website contact form and retain data for 6+ months

1. In January 2026, John Smith uses the website to fill out a form to contact one of our associates. Nothing in AEP is currently set up when this happens other than his pseudo-identifier is collected. There is no persistent/reliable identifier such as CRM ID or any other known ID.
2. In July 2026 (six months later), John becomes a client. He is assigned with a CRM ID, which is a valid identifier in the Client Dataset. His profile will now exist and his profile attributes will be ingested.

We would like to know what actions John did on the website before he became a client; e.g. he read articles, he searched for an associate five times before he contacted one, he looked at products, etc.

Challenges:

1. Prospect records are only retained for 25 days, which is far too short for our use-case. Also note that as previously mentioned, Prospects profiles aren't set up.
2. We are not collecting any identifiers for prospects. There is PII available to be collected when a prospect fills out the contact form. However, our Privacy Policy might not allow us to collect any PII (needs to be confirmed). Is collecting email or some other PII absolutely necessary? Are there any other workarounds to avoid using PII collected from the website?

How can we set up AEP for this use-case? Thank you.