NGINX Optimization for Server-Sent Events (SSE)

I have developed a group chat app using Node.js and have load tested with Gatling. When I test from either my laptop or directly on my droplet, it can handle 382 connections. However, when I set Gatling in the droplet to bypass NGINX and connect directly to Node.js, it can handle more than 8,000 connections.

Any ideas why NGINX maxes out at precisely 382 connections?

Here is my sites-available:

server {

  server_name chat.quadspace.tv;
  listen 443 ssl http2;
  listen [::]:443 ssl http2;
  ssl_certificate /etc/letsencrypt/live/chat.quadspace.tv/fullchain.pem; # managed by Certbot
  ssl_certificate_key /etc/letsencrypt/live/chat.quadspace.tv/privkey.pem; # managed by Certbot
  include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
  ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

  location / {

    # https://serverfault.com/questions/801628/for-server-sent-events-sse-what-nginx-proxy-configuration-is-appropriate

    proxy_pass http://localhost:3000;
    proxy_http_version 1.1;
    proxy_set_header Connection '';
    
  }

}

server {

  if ($host = chat.quadspace.tv) {
    return 301 https://$host$request_uri;
  } # managed by Certbot

  server_name chat.quadspace.tv;
  listen 80;
  return 404; # managed by Certbot

}

And the headers sent from Node.js:

res.writeHead(200, {
  'Content-Type': 'text/event-stream',
  'Connection': 'keep-alive',
  'Cache-Control': 'no-cache',
  'X-Accel-Buffering': 'no'
});