GrapheneOS plans to eventually move away from Linux if you read their roadmap: https://grapheneos.org/faq#roadmap
To get an idea of the near term roadmap, check out the issue trackers. The vast majority of the issues filed in the trackers are planned enhancements, with care taken to make sure all of the issues open in the tracker are concrete and actionable.
In the long term, GrapheneOS aims to move beyond a hardened fork of the Android Open Source Project. Achieving the goals requires moving away from relying on the Linux kernel as the core of the OS and foundation of the security model. It needs to move towards a microkernel-based model with a Linux compatibility layer, with many stepping stones leading towards that goal including adopting virtualization-based isolation.
The initial phase for the long-term roadmap of moving away from the current foundation will be to deploy and integrate pKVM and CrosVM, reinforcing existing security boundaries. The Android Open Source Project has been making significant progress on this front, so our next milestone is to securely deploy Android apps using this virtualization setup. In the longer term, Linux inside the sandboxes can be replaced with a compatibility layer like gVisor, which would need to be given a new backend alongside the existing KVM backend. Over the longer term, i.e. many years from now, Linux and the usage of virtualization can fade away completely. The anticipation is that many other projects are going to be interested in this kind of migration, so it's not going to be solely a GrapheneOS project; this is demonstrated by the current existence and development of gVisor and pKVM, as well as various other projects working on virtualization deployments for mobile. Having a hypervisor with verified boot still intact will also provide a way to achieve some of the goals based on extensions to Trusted Execution Environment (TEE) functionality even without having GrapheneOS hardware.
Hardware and firmware security are core parts of the project, but it's currently limited to research and submitting suggestions and bug reports upstream. In the long term, the project will need to move into the hardware space.