[zehner]

The issue is that my kodi machine is limited to about 5400kbps (5Mbps) on a 1Gbps network with 150Mbps to WAN.

What kernel do you use ("uname -a")?

Older kernels are known to have problems with the rtl8111 chips. I have the rtl8111h and its a desaster. Newer kernels (>= 5.3) should be better, but I'm still on LibreELEC 9.2 and I think not all changes are backported to the 5.1 kernel, because I still have this problem (but much less frequently than with older 4.19 kernels). You can try the known workarounds (https://kodi.wiki/view/Chromebox => search for "slow ethernet performance" on the page), perhaps they still work.
KODI has an excelent caching mechanism, so most of the time it hides this problem for you. But for playback types where caching isn't implemented (LiveTV, streams) you can't ignore, how shitty those cheap realtek chips are.