Has the original poster tried the Packet Steering option? I actually don't know where this lives on the command line, in Luci its found under the Network -> Interfaces -> Global Network Options tabs, and then a checkbox. It enables packet handling to get spread across CPU cores. Another thing to try would be to install irqbalance, (need to enable it after package install, OWT website has a page on that) which does something similar. Sometimes, it's beneficial to do both.
Since you have a dual core CPU, and cake is a single threaded process, it should help if some of the rest of the load gets spread more evenly. You might find a higher limit threshold than you have now. How much, I couldn't say offhand. It isn't going to get you to 500mbit.
Otherwise, to fully utilize the speed of your link, you probably want to look into something with more CPU power to handle the load. This thread will point you in a good direction, with solutions that have already been suggested. https://forum.openwrt.org/t/so-you-have-500mbps-1gbps-fiber-and-need-a-router-read-this-first/90305