Could you try enabling the in-game option ‘Smart Network Packets’ in the ‘Network’ panel and see if this has any positive effect? (Note, this can’t be used in combination with the ‘Player List Plus’ mod, as they conflict.)
If not, you’ve mentioned trying a few solutions already, but can I confirm if you have tried all possible solutions in this article?
I’d recommend running through as many of these, where possible.
Let me know how you get on!