We currently have a suite of webdriverio/appium tests for our react native app, with separate wdio.conf files for local, simulator, BrowserStack etc.
All of the examples I can find for device farm either involve first uploading the test suite as a zip up front, or creating a custom test runner that obtains a selenium grid url from Device Farm and then triggers the wdio command (but I can only see this being used for Selenium, nothing for appium/app testing).
Is it not possible to run tests from my local machine whilst pointing to AWS Device Farm devices?
Unfortunately this is not possible with AWS Device Farm, as you have already noticed that they do not have a Selenium Grid for mobile devices. In general for me it has been a huge hassle to get Appium tests to work on AWS using CI/CD - both because of the setup and the inability to easily debug since I can't run the tests locally against their devices. I would advise using another cloud farm service like Browserstack which allows you to target their grid from your local machine