Websocket Cloudflare with Nginx 520 error - node.js

Question

I am jumping in on a project with some socket issues over SSL and Cloudflare... I know.. I have read about 50 different stack overflow posts and 200 blog posts to try to figure this out. The project works on my local dev server/computer just fine...
I think I am on the right track - But could use some help/pointers if ya'll can.
First, I thought it was weird that the /socket-io/ proxy_pass was at port 6379, the same as redis... Maybe it should be? When this was set at 6379, the socket connection will not connect - With or Without Cloudflare enabled ( I paused cloudflare to test this out).
I read through the express server and saw that the socket server seems like it's linked to the express server at port 4000... so I changed the proxy_pass for /socket-io/ to port 4000 and it reconnects. This works with Cloudflare paused/running... so maybe it's not cloudflare after all. Still, even though it says the socket has reconnected in the browser, nothing is working.
I'll start by sharing my NGINX config - Let me know what else ya'll need to see, please. Thanks for taking your time to help me out/pointing me in the right direction! I really appreciate learning about this stuff.
server {
listen 443 ssl;
listen [::]:443 ssl;
server_name dev-app.myapp.com;
location / {
root /var/www/myapp_frontend/build/;
try_files $uri $uri/ /index.html;
#proxy_pass http://localhost:8080;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $host;
}
location /api/ {
proxy_pass http://localhost:4000/;
include /etc/nginx/proxy_params;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
}
location /socket.io/ {
proxy_pass http://localhost:6379;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_read_timeout 86400;
}
location ~ \.php$ {
include snippets/fastcgi-php.conf;
fastcgi_pass unix:/run/php/php7.0-fpm.sock;
}
location ~ /\.ht {
deny all;
}
ssl_certificate /etc/letsencrypt/live/dev-app.myapp.com/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/dev-app.myapp.com/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
}
server {
if ($host = dev-app.myapp.com) {
return 301 https://$host$request_uri;
} # managed by Certbot
server_name dev-app.myapp.com;
listen 80 default_server;
listen [::]:80 default_server;
return 404; # managed by Certbot
}
Edit-1
I did see that cloudflare requires certain ports... Am I wrong to think that these ports only refer to the initial listening port, for example 443 above, since the proxy_pass ports are all using localhost?