So, I think the first thing I have asked you is " do you use virtio nic in your vm ? (don't use e1000 or realtek)"
don't use e1000 in production, never, only use virtio-nic.
Sorry, I don't have time anymore to help you, tI'm oo busy at work currently. Seem that you are alble so reach more than 1gbit/s with multiple parallel connections. Still don't known, but you should be able to reach 6-7 gibit/s wihout problem with 1 connection with virtio on both size.
good luck !