Yep, you found the same that I did. Fwiw - Peplink only supports UPNP on the untagged LAN unless the packet has a TTL higher than 1.
Keep your head up. You are on the right track. You have the best dual wan product you can get. As time goes on - it is only get more feature rich and efficient. That is the beauty of a company that provides firmware updates regularly.
If queue management and congestion control are important to you - look in the feature requests section of the forum. Find a thread that requests that feature and then chime in on it. I don’t know what weight the engineers put on the threads comment count, but chances are that it does play some kind of factor in their decision making process. There are others that want layer 7 QOS, others want coordinated handoffs from WAPS, some are asking for easier very deployments, and probably the most requested is the support for OpenVPN. If you don’t find someone else asking for it - start up a new one.
Take a look at this post by @DocPecos - it may be able to help your situation.