Maybe I am missing the explained distinction ("Think Fastlane but for the dev portal"), but fastlane also supports the listed examples in the dev portal[1][2].
Yeah Fastlane supports the two. I haven't looked into the implementation yet, but I supposed that Fastlane is based on the existing Apple API, for example, https://developer.apple.com/documentation/appstoreconnectapi for listing registered devices.
However, there are many other operations that Apple doesn't provide API for. For example, it would save me a lot of time if Apple provides an API to update provisioning profiles. I guess operations like these can only be automated with say Chrome Puppeteer.
For the "tedious" things that I need to do, usually they are outside the scope of Fastlane, and that's why I'm building a new thing
[1]https://docs.fastlane.tools/actions/register_devices/ [2]https://docs.fastlane.tools/actions/#code-signing