Nginx try_files redirect trailing slash and 403 Forbidden

I am trying to create a combined "super cache" -> file -> module pattern with try_files ("super cache" term taken from wordpress super cache as it is a similar method).

The files that are applicable for caching are located inside the "cache" folder.

The file should be served if it exists in one of these locations.

1. /$cache$uri
2. /$cache$uri/
3. $uri
4. $uri/

1.cache -> 2.file -> 3.pattern

    location / {
        try_files /$cache_dir$uri /$cache_dir$uri/ $uri $uri/ /index.php;
    }

Example with address 'domain.com/css/style588.css'

1. look for /srv/www/domain.com/public_html/cache/css/style588.css
2. look for /srv/www/domain.com/public_html/cache/css/style588.css/index.html & .php
3. look for /srv/www/domain.com/public_html/css/style588.css
4. look for /srv/www/domain.com/public_html/css/style588.css/index.html & .php
5. if not found : /srv/www/domain.com/public_html/index.php

I have tried to make it work with the config below. What am i doing wrong?

included domain.conf from nginx.conf

server {
    ## index index.html index.php (defined in http block, works)

    server_name domain.com;
    root        /srv/www/domain.com/public_html;

    ## cache directory ##
    set $cache_dir 'cache';

    ## no cache if post ##
    if ($request_method = POST) {
        set $cache_dir 'null dir';
    }

    ## 1.cache -> 2.file -> 3.pattern ##
    location / {
        try_files /$cache_dir$uri /$cache_dir$uri/ $uri $uri/ /index.php;
    }

    ## .php (set cgi.fix_pathinfo=0 in php.ini, especially if php is on another machine) ##
    location ~ \.php$ {
        try_files $uri =404;
        include /etc/nginx/fastcgi_params;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

Examples i tried with above config

domain.com: (403 Forbidden)

public_html/index.php is not served.

–

domain.com/non_existing_folder: (OK)

public_html/index.php is served.

–

domain.com/folder_exists_in_cache/: (OK)

public_html/cache/folder_exists_in_cache/index.html is served.

–

domain.com/folder_exists_in_cache: (redirect) (no trailing slash)

Redirect header 301 permanent.
public_html/cache/folder_exists_in_cache/index.html is served.

but url displays as domain.com/cache/folder_exists_in_cache/

–

uri domain.com/cache/existing_empty_folder: (403 Forbidden)

public_html/index.php is not served.

–

How do i avoid 403 forbidden (and the redirects when there is no trailing slash but the folder exists in cache?)

Have tried setting permissions on folders, with the same result.

Thanks, for your help!