Today I had some spare time and devoted it to testing this further for almost three hours.
You can see here that when Flow Control was disabled on the R7800 built-in switch I couldn't get uninterrupted LAN traffic.
In the next tests I've used R7800 with default switch settings (Flow Control is Enabled).
This time I decided to disable the Flow Control of the PC NIC (Realtek) as I posted here.
As you can see from the post I didn't get the uninterrupted LAN traffic with Belkin RT3200.
For my latest tests I've used R7800 but I've disabled Flow Control of the PC Realtek NIC. As @moeller0 suggested
To my surprise this time I was able to get full WAN speed on the PC while it was streaming the same 4K movie stream (I've used for the previous tests) simultaneously to three 100Mbps devices (two 4K AndroidTV boxes and a 4K Smart TV) connected to second gigabit switch and one Laptop, connected as a 100Mbps device to the R7800 built-in switch, which was running iperf3 session to the PC server.
This way I had four simultaneous LAN transfers all from the PC to four different 100Mbps devices. All LAN traffic was going almost completely uninterrupted. I had only 4 or 5 single occasional ping losses to the 100Mbps devices for more than 30 minutes while I was running numerous (more than 50) WAN speed tests on the PC at full WAN speed. I didn't observe any LAN traffic interruption during this long test session.
With this setup I can say that my task is 99% possible.
I doubt that I can surely say the issue is completely resolved only because there are different hardware combinations that still cannot complete the goal of uninterrupted WAN/LAN traffic at full duplex speeds.
Any other thoughts from anyone here on the subject?