I came here to ask this same question. I’m using M365 for email. I don’t want a service that will mess with my DNS or MX records. I just want something that will capture my DMARC email reports and display them in a useful, human readable way. I found a wide variety of options on https://dmarcvendors.com/ including:
At my last company we worked with Dmarcian and I enjoyed the process. We had weekly calls to help identify mailboxes that were sending on our behalf and to identify the correct person/department to contact to set up the proper SPF/DKIM records. We started at p=none and when I left they were close to going to p=quarantine, though I think the project stalled after I was gone so I’m not sure if they’re still making progress on it or not.
Valimail has a good reputation, but was at least 2x the cost of what Dmarcian quoted us for.