You don’t have to compile GrapheneOS from source thanks to avbroot which can take a release zip, patch it with root and sign it using a custom avb key. This lets you root while keeping the bootloader locked.

The rooted-graphene project automates this completely using GitHub actions. It even lets you do OTA updates like normal GrapheneOS.

I am able to access the data folder which is located in Internal storage/Android/media/com.whatsapp. This is without root using the AOSP files app, not sure if that’s what you meant.