Pixel 7a and Pixel Tablet since yesterday here.

On Pixel 9 stock they mention you can disable AIcore in the settings, so I would assume GrapheneOS is unaffected.

How much is a kWh in your parts? Noise, ambient temperature? You can buy very decent refurbished Lenovo tiny PCs with some 16 GB RAM and 6 cores and half a TB SSD which will run Proxmox and are low power and noise. You can go multi-node Proxmox later if you want to expand. k8s and related are also an option.