Are you running VPNs *through* the Draytek or terminating tunnels on the Draytek?

From what I can see, it seems the router is working as intended.

What would I do?

If everything is working as intended in the office and no one is complaining, I would leave it alone and concentrate on making money .

I would also read this page and make sure hardware acceleration was working, leaving it on auto. Make sure Data Flow Monitor, QoS and Bandwidth Limit is disabled. This will give you the best chance of making use of the 500Mb.
https://www.draytek.com/support/knowledge-base/5308