8th gen Pixels have the capability to do DP alt mode, though stock OS currently disables it by default, and GrapheneOS hasn't enabled it yet, more info here:
https://discuss.grapheneos.org/d/8244-pixel-8-displayport-support/3
That's the first building block, so if you're interested in this functionality, you need to get an 8th gen Pixel as a starting point.
It is likely that one of the reasons for Google to disable DP alt mode on the new Pixels right now is because they want to finish their desktop mode first, and they have been working on it - just very slowly.
There might come a time when AOSP has a proper DeX alternative - it's too early to tell. This isn't something that GrapheneOS would develop on their own, so it will have to come from upstream.