Yes they did. I reckon that's when the buffering issue is likely to have begun.They changed stream providers from stream motion to some other company. I believe thats where all the issues lie.
I have an ax5400 dual band router and the TV is actually plugged in to a cat-6 Ethernet cable anyway (Chromecast built-in). I can get 4k from other sources beautifully.I would probably check your Wi-Fi to ensure it has dual band.
It makes a huge difference in terms of reliability and speed without choosing to go with ethernet (although most houses can only do 100 mbits best case through powerline).
Anyway, NBN guy is here now so FTTC will be replaced by FTTP.