Since you seem to be one of the developers, one thing that I wish Graphene focused on more is browser fingerprinting. This is is probably the number one threat against privacy nowadays. Vanadium is very usable, but it seems to be quite easily fingerprintable.
The founder, afaik, not just a developer.
Tor Browser seems to be a project that requires multiple full time developers. I don't think GrapheneOS have the resources right now to do this alongside their OS development, device support and app overhaul plans.
Also please don't take this as any criticism of your suggestion, but there have been multiple 'privacy' browser projects based on Chromium for Android. It's a little frustrating that they couldn't collaborate some base like this to the open source community.
> 'privacy' browser projects based on Chromium for Android
As far as I know none of these projects have tackled the JS fingerprinting problem. The most earnest attempts seem to be Brave and Firefox with the Arkenfox user.js, but they have their own problems. The basic issue is that JS gives websites far too much control over the user's device. The JS spec should have never allowed websites control over the clipboard (e.g. to disable paste), to know if the user is active, when the mouse is being moved, etc. Since it is too late now, short of disabling JS entirely, there will be usability tradeoffs, but I think these are necessary (at least optionally) in an OS like Graphene.
Unfortunately, browsers have often done too little, too late when it comes to privacy. For example, until recently, most browsers allowed third party cookies by default.