It’s good that you’ve back up and running, but you haven’t found out why your network is performing so badly. Receive speeds of 500 Mbps and send speeds of 650 Mbps are too low. (Iperf3 measures the low-levell network speed, irrespective of what data transport layer you’re using on top.) As @fzinken said, the network speed should be around 920-930 Mbps both ways.
Clearly something is still wrong on the network side and by moving to a kernel-based mount you have managed to mitigate the problem, but not fix it. It might, for example, bea limitation of the way Kodi 19 has been configured to use libnfs that has revealed the underlying network problem. I came across this thread, admittedly five years old, that discusses such matters.