Hi Matt,
I understand you are concern about if any DNS records reject your email server and you are not able to test it by changing the SPF record.
After the name servers switched to Office 365, the required DNS records were created by Office 365 and cannot be change by customer themselves. However, to point name servers to Office 365 hosts is not a necessary requirement. Here is an article about Add your domain to Office 365 but continue to manage your domain's DNS records providing detailed information about it, you can manage your domain DNS records on your side at first and see if the issue persists?
Thanks,
Maggie Li