Personally I'd go with a third-party service that will manage the PKI side of things, possibly two. That would relieve you of a big burden, leaving you to only invoke their APIs as needed. Most big CAs have specific IoT products.

On the do-it-yourself side, take a look at Google's Certificate Authority Service https://cloud.google.com/certificate-authority-service and the AWS Certificate Manager Private Certificate Authority https://aws.amazon.com/certificate-manager/private-certifica...

Another choice is EJBCA, and here's their documentation for the IoT use case: https://doc.primekey.com/ejbca/solution-areas/iot-and-device... EJBCA is open source, but at least some of the IoT features (specifically those that deal with device enrolment) are enterprise-only.