I don't see a RF ground. The (-)DC voltage is not a ground. On most power supplies it is a common. If connected to ground at the supply, it is connected to mains electrical system ground, not a good RF ground.
Since you are in a metal chassis, remove the paint to have a good connection between the chassis and all the connectors including the 4 PCB stand-offs. Then bond the chassis to your RF ground.
Also make sure you have a proper size fuse inline between your power source and your uBITX.
The green/yellow/orange bundle to your volume control: Move that forward and down close to the chassis to minimize coupling from the encoder and USB