A quick follow up: While waiting for the Cable Matters adapter to arrive and inspired by posts to this thread, I played around with WiFi. Specifically I moved a redundant Airport WiFi router to the end of the cable previously connected to the TV and set up a new network. With the TV connected to the new WiFi network (at a distance of about 2 metres) I got speeds of over 200Mbps download as reported by the WebOS speed check tool and 50Mbps upload. Mysteriously, Speedtest in the WebOS browser still reported only (variable) 40-90 Mbps download and 200 Mbps upload. Finally, and again based on hints in this thread, I found the Netflix speed check and that reports 280Mbps. For comparison, my iPhone 12 connected to the same WiFi reports 400Mbps up and down.
Tentative conclusion. (1) Software is playing a role, (2) LG seems to have deprecated Ethernet in favour of WiFi, (3) I now have satisfactory speed even if the Cable Matters adapter still doesn't work. I'll still test the adapter when it arrives and put results here.