Cloud They publish the alpha/beta builds on their Telegram.
The Github releases method does require you to manually monitor, download, and update your apps, so that is not ideal, but it can be a better than solution than F-droid, as F-droid has many issues.
A GrapheneOS community member is creating a new app store that's focused on privacy and security which will have unattended updates called Accrescent. It is currently in early stages, but is actively being developed, so that's definitely something to look out for.
Additionally, keep in mind that the GrapheneOS team plans to eventually do their own builds of 3rd party open source applications that will be downloadable from the "Apps" application that comes pre-installed on GrapheneOS, so that's also something to look out for in the future as another option with unattended updates!