So I downloaded the beta firmware for the router, which enables flow control. As soon as I enabled it, it worked great with devices directly connected to the router. As soon as I plug in the unmanaged switch, it goes right back to 1800Mbps on 2.5 NIC PC, and 50-100Mbps on the gigabit NIC PC.
Now I am wondering, even though they are unmanaged switches, are they somehow interfering? I mean obviously there was an issue before, but the flow control fixed it until the switches were involved again.
Google removed the ability to turn them into tiny planets YEARS ago, and that's what upsets me the most.