"any plastic router has some sort of QoS build in"... that doesn't work.

If you don't have an expensive product, do some tests with and without the router's QoS. You'd be better of with a raspberry Pi doing QoS in most cases.

Agreed. Found that none of the routers I've ever bought came with QoS that actually worked, or at least made a noticeable difference.

Even if you do have an expensive product, the QoS in the stock firmware is guaranteed to have major problems.