@frozenpea I can’t make or influence the decision of an out-of-warranty exchange, but what do you think of the idea of bypassing the built-in network interface with a cheap USB gigabit adapter? See also Gigabit Ethernet Adapter for Vero 4K - #5 by JimKnopf (I bought it in 2019). But not all adapters will work, so you have do some research here in the forum.
Addition: Just tested it again after a long time directly connected to a Vero4k+ without an USB-hub and a Windows 10 PC as the iperf3 counterpart.
This adapter is good for 280 Mbit/sec sending and 340 Mbit/sec receiving data.
# dmesg|grep -i ax88
[ 280.883259] usb 1-1: Product: AX88179
[ 281.301229] ax88179_178a 1-1:1.0 eth1: register 'ax88179_178a' at usb-xhci-hcd.0.auto-1, ASIX
AX88179 USB 3.0 Gigabit Ethernet, 00:50:b6:28:ea:e4
[ 281.301761] usbcore: registered new interface driver ax88179_178a