It turns out that a2dp is hogged by the first pulseaudio instance. Which is started by gdm and is not configurable. First, there are '~gdm/.pulse/client.conf' and '~gdm/.config/pulse/client.conf'. Modifying these files doesn't prevent pulseaudio from starting. Second, there is /etc/xdg/autostart/pulseaudio.desktop. Overriding it with ~gdm/.config/autostart/pulseaudio.desktop doesn't work. These were the only solutions I could find via google. Turns out that pulseaudio.service was started by pulseaudio.socket, which was required by sockets.target. I had deleted the .wants symlink and had enabled the pulseaudio.socket manually for my user. Of course all of this is unnecessary when using another login manager.