patrickedwards Hi there, as you have discovered you need to install a STT or voice typing service.
There are two options that I'm aware of depending on whether or not you're happy to use Google.
The service which works the best is Google voice typing. It makes sense to use it if you are already using GBoard as a Keyboard. The voice typing comes packaged in the Google app which you can install from Play Store or Aurora Store. https://play.google.com/store/apps/details?id=com.google.android.googlequicksearchbox
It'll need network permission at least to install the language packs. Once installed, tap the microphone icon in top right of GBoard keyboard, and voice typing will work.
The second option integrates fine with the default GrapheneOS Keyboard but is less feature rich. It supports less languages, and does not type periods, but it still does the basics. That's Sayboard and you can download the APK for it from GitHub here:
It requires internet permission to download language pack initially but can be disabled after. Then when you tap microphone icon in the default stock Keyboard it will load Sayboard and you'll be able to start voice typing.