Bookstack setup with Nginx for SSL on custom port

Hi,
My ISP blocks port 443 so I have nginx listening for SSL on port 444. I’m having some trouble setting up SSL for bookstack and accessing via the internet. I’m new to docker and self-hosting, so some help would be greatly appreciated!

My docker-compose file is this -

version: "2"
services:
  bookstack:
    image: lscr.io/linuxserver/bookstack:latest
    container_name: bookstack
    networks:
      - nginx
    environment:
      - PUID=1000
      - PGID=1000
      - APP_URL= http://192.168.1.165:6875
      - DB_HOST=bookstack_db
      - DB_USERNAME=bookstack
      - DB_PASSWORD=<redacted>
      - DB_DATABASE=bookstackapp
    volumes:
      - ./config:/config
    ports:
      - 6875:80
    restart: unless-stopped
    depends_on:
      - bookstack_db
  bookstack_db:
    image: lscr.io/linuxserver/mariadb:latest
    container_name: bookstack_db
    networks:
      - nginx
    environment:
      - PUID=1000
      - PGID=1000
      - MYSQL_ROOT_PASSWORD=<redacted>
      - TZ=Europe/London
      - MYSQL_DATABASE=bookstackapp
      - MYSQL_USER=bookstack
      - MYSQL_PASSWORD=<redacted>
    volumes:
      - ./config:/config
    restart: unless-stopped

networks:
  nginx:
    external: true

and my nginx proxy host file is this -

server {
  set $forward_scheme http;
  set $server         "192.168.1.165";
  set $port           6875;

  listen 80;
listen [::]:80;

listen 444 ssl http2;
listen [::]:444 ssl http2;

  server_name bookstack.domain.tld;

  # Let's Encrypt SSL
  include conf.d/include/letsencrypt-acme-challenge.conf;
  include conf.d/include/ssl-ciphers.conf;
  ssl_certificate /etc/letsencrypt/live/npm-11/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/npm-11/privkey.pem;

# Asset Caching
  include conf.d/include/assets.conf;

  # Block Exploits
  include conf.d/include/block-exploits.conf;

    # Force SSL
    include conf.d/include/force-ssl.conf;

proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $http_connection;
proxy_http_version 1.1;

  access_log /data/logs/proxy-host-8_access.log proxy;
  error_log /data/logs/proxy-host-8_error.log warn;

  location / {
   
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection $http_connection;
    proxy_http_version 1.1;
    
    # Proxy!
    include conf.d/include/proxy.conf;
  }

  # Custom
  include /data/nginx/custom/server_proxy[.]conf;
}