Nginx settings to return statics with a large number of connections?

Question

Nginx

I have a simple vps (1 CPU Core @ 3.9Ghz+, 1GB DDR4 RAM, 20GB NVME). Installed Centos 7 64 and Nginx. This virtual server is used to upload several files (1-2 GB) that are downloaded via a direct link. Constant load (sudo netstat -anlp | grep :443 | grep ESTABLISHED | wc -l) - 400-500. The server is loaded somewhere by 30-40% (CPU and RAM). But at the same time, a 503 error is issued with a constant frequency. Then it passes (apparently, when the number of connections decreases), then again 503 and so on constantly.
nginx config:

user  www www;
worker_processes auto;
error_log  /www/wwwlogs/nginx_error.log  crit;
pid        /www/server/nginx/logs/nginx.pid;
worker_rlimit_nofile 51200;

events
    {
        use epoll;
        worker_connections 51200;
        multi_accept on;
    }

http
    {
        include       mime.types;

    include proxy.conf;

        default_type  application/octet-stream;

        server_names_hash_bucket_size 512;
        client_header_buffer_size 32k;
        large_client_header_buffers 4 32k;
        client_max_body_size 50m;

        sendfile   on;
        tcp_nopush on;

        keepalive_timeout 60;

        tcp_nodelay on;

        fastcgi_connect_timeout 300;
        fastcgi_send_timeout 300;
        fastcgi_read_timeout 300;
        fastcgi_buffer_size 64k;
        fastcgi_buffers 4 64k;
        fastcgi_busy_buffers_size 128k;
        fastcgi_temp_file_write_size 256k;
    fastcgi_intercept_errors on;

        gzip on;
        gzip_min_length  1k;
        gzip_buffers     4 16k;
        gzip_http_version 1.1;
        gzip_comp_level 2;
        gzip_types     text/plain application/javascript application/x-javascript text/javascript text/css application/xml;
        gzip_vary on;
        gzip_proxied   expired no-cache no-store private auth;
        gzip_disable   "MSIE [1-6]\.";

        limit_conn_zone $binary_remote_addr zone=perip:10m;
    limit_conn_zone $server_name zone=perserver:10m;

        server_tokens off;
        access_log off;

server
    {
        listen 888;
        server_name phpmyadmin;
        index index.html index.htm index.php;
        root  /www/server/phpmyadmin;

        #error_page   404   /404.html;
        include enable-php.conf;

        location ~ .*\.(gif|jpg|jpeg|png|bmp|swf)$
        {
            expires      30d;
        }

        location ~ .*\.(js|css)?$
        {
            expires      12h;
        }

        location ~ /\.
        {
            deny all;
        }

        access_log  /www/wwwlogs/access.log;
    }
include /www/server/panel/vhost/nginx/*.conf;
}

Tell me what to fix in the nginx config to get rid of 503 errors, even with a large number of simultaneous connections / downloads.