Cool project. But in practice, you have created several ground loops. It is similar to the old school method where the chassis is used as the ground conduit. If the design is not a high gainer, it'll probably still work fine. I've milled several hundred boards using a specialty machine designed for the purpose. You have overcome the cutting depth problems nicely with your equipment. There really is no added ground plane capacitance. There would need to be a second plane, not ground, that is a few mils away from the ground plane in order to have significant board capacitance. That means a multi-layer fabrication. That's a high frequency decoupling technique, and doesn't really apply to audio. Anything that makes a design clean and still work is game to me. Looking forward to the rest of your project.