I was recently trying to do something similar so I feel your pain. What I can tell you is that at least as far as I can tell (I'm still experimenting) this is definitely possible…but information on how to do it is slim to none. In part it's because it's difficult and time consuming to fully explain. I don't have enough time to sort through all the details with you but maybe the summary below will give you enough to get it going...
What you need to know is that you can have parent and child queues where children inherit all of the bandwidth restrictions from the parent in addition to more restrictions you might specify. So for EACH VLAN interface you will want a default queue (probably whose parent is the default queue). You will then want something like "qInternet" which is a parent queue for "qAck", "qHighPriority" and "qBulk" or any other queues you want.
You will then setup qInternet to have your 20Mbps cap by setting the upperlimit m2 to 20Mb (I'm assuming you are using HFSC) which will put a hard cap on that VLAN at 20Mb for traffic in qInternet or it's children. Inside of the child queues of qInternet you can specify hard limits (upperlimit) or portions of a congested link (link shares) for each of the child queues.
Now you need to assign traffic to the queues. Do this using floating firewall rules. Set the action to "Match", the interface to your WAN interface (so you are limiting only packets originating from the WAN), the direction to "any", the Ackqueue/Queue to "qAck"/"qBulk" and you're set. This should place all your Internet traffic into the bulk queue (and the Ack queue). You can verify this on the queue status page (you may need to clear out your state table first). Now you can create additional similar floating rules except also define ports, etc. for them to put your high priority traffic into your high priority queue.
