Hey, I use GrapheneOS daily on a Pixel 6 Pro (My 7 Pro still shipping), and I host my own WG VPN. It doesn't drain much battery, but I think it can still be improved by integrating WireGuard straight into the kernel. The WG Android app will automatically use it without any other modifications and it would be a great addition considering the theme of the OS is privacy and security. The implementation is already complete (aka stable, marked as so on the website) and the repo already has a script to integrate it automatically. I'm not sure if any further modifications are required (I'm not an expert in Kernel programming or programming in general, just a hobby coder for the most part) but to me it just looks like a quick drop in.
Here's the repo for the implementation, if you could implement it, I would be very grateful! (And here's the Android app)