Hmm, I think at this point I'm just guessing. My experience with traffic shaping is limited (pun intended!).
Now that you have setup an ExpressVPN connection and have routing correctly configured perhaps you should start a new thread in the traffic shaping sub-forum.
Informed guess work follows...
There is relatively little documentation about this and what there is mostly relates to 1.2.X (the definitive guide). It's worth thinking about what point in the chain the vpn encryption happens in combination with how the traffic shaper works with queues.
The inbound queue is working because that actually limits traffic as it leaves the LAN interface. At that point it is unencrypted VOIP traffic (UDP on whatever port you are using). However the outbound queue is liming traffic leaving the box which is encrypted TCP probably on port 1194. Further confusing matters is that this traffic leaves as an encrypted stream on WAN but it also leaves on ExpressVPN, is it encrypted at this point?
Since you have added rules on ExpressVPN which are catching traffic leaving that must provide a clue.
You must have some catch-all rule sending stuff to qp2p or is that the default queue?
Like I said guesswork!