DAVx5 works well, but it is indeed rather surprising that Graphene does not come with a calendar or an email client. I guess the idea is that you can download F-Droid and choose your own, but even F-Droid is not provided by default.
On bundling apps in general: https://grapheneos.org/faq#bundled-apps. For the calendar app particularly I think they assessed that the AOSP Calendar app was beyond saving (left to rot by AOSP/Google). I cannot remember if they still have plans to develop a calendar app.
I believe you're right that the idea is for people to download the apps they want (from wherever they choose). GrapheneOS has a complicated history with F-Droid though. Unfortunately, unless their approach was different in a lot of significant ways, it is unlikely GrapheneOS will include F-Droid in their Apps app repository.
