I have a pi CM 4, and with irqbalance and packet steering enabled I get a peak cpu use around 55 to 60% when doing SQM layer cake on a 1000/40 Mbps connection during a buffer bloat test.
As ypu probably already know, the Pi's built in ethernet is 1000Mbps only, so 1400 is not really an option, realistically. In theory you could get a CM4 and the IO board with the PCIe slot, and plug in a dual port 10/5/2.5 Gbps ethernet NIC, but I doubt that the 1X PCIe slot practically has the bandwidth available that is needed to make that work well.... (1400/60 however might work). I am not sure that 2.5 Gbps USB3 ethernetbdongle exist that are supported by Linux out of the box, but IIRC people attempted to use those by compiling their own drivers into their OpenWrt images....
Well a raspberry Pi 4B with a decent USB3-Gigabit-ethernet dongle (and the proper configuration) should allow for that.
I believe this post might help? That whole thread seems relevant....
No idea, but the R4S package seems nice, price-wise it is close to a RPi4B (with case and USB dongle)
pros: second ethernet via PCIe (possible with the CM4, but )
cons: only 2 instead of 4 A72 cores (I am not sure the 4 A53 cores can make up for that, at least SQM is more latency bound then throughput bound)
What ever you end up using, would be nice to post your final decision and how well it works... Thanks in advance.
I understand the desire for a single board solution, but if you end up playing with the current RPI4, this thread might be helpful, it includes the discovery of a "good" usb3 ether interface (TP-Link UE300, realtek chip) like Moeller0 mentions, that uses far less resources than some other types that will bottleneck you. There's a few other good threads are out there on Rpi4 use, as well.