What policy should I choose first?

Choose p=none if you are just starting. Move to quarantine or reject after monitoring.

Should I use strict alignment?

Strict alignment is stronger, but relaxed alignment is easier during migrations with multiple mail systems.

Use a mailbox or DMARC platform address that can process aggregate XML reports.

GEN tool

Create a starter DMARC DNS record that can be copied into your DNS provider.

SenderDNSReceiver

GEN

Enter a domain to run the check.

Publish the host as _dmarc.yourdomain.com and the TXT value exactly as shown. Start with p=none when you are still discovering all senders.

A typical migration is p=none, then quarantine at a partial percentage, then reject after reports confirm legitimate mail is aligned.