NGINX: Return CORS headers when request method is OPTIONS and file exists, otherwise pass to PHP-FPM

I have an NGINX with a PHP-FPM instance behind it. OPTIONS requests for paths for which a file exists in the file system should be handled by NGINX. For those requests, NGINX should return Access-Control-* CORS headers. OPTIONS requests for which no file exists should be passed to PHP-FPM.

The logic should be something along those lines:

    location / {
        # In this case: Check if file exists
        # - yes: return CORS headers
        # - no: pass request to PHP-FPM
        if ($request_method = 'OPTIONS') {
            # This causes an error
            try_files @cors;
        }

        # Normal request handling for all non-OPTIONS requests:

        # Try to serve file directly, fallback to index.php if file does not exist
        try_files $uri /index.php$is_args$args;
    }

    location @cors {
        if (-f $request_filename) {
            more_set_headers "Access-Control-Allow-Credentials: true";
            more_set_headers "Access-Control-Allow-Origin: example.com";
            more_set_headers 'Access-Control-Allow-Methods: POST, GET, DELETE, PUT, OPTIONS';
            more_set_headers 'Access-Control-Allow-Headers: content-type,ngsw-bypass';
            more_set_headers 'Access-Control-Max-Age: 3600';

            more_set_headers 'Content-Type: text/plain; charset=UTF-8';
            more_set_headers 'Content-Length: 0';

            return 204;
        }

        try_files /index.php$is_args$args;
    }

    location ~ ^/index\.php(/|$) {
        fastcgi_pass localhost:9000;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;
        include fastcgi_params;

        fastcgi_param REQUEST_METHOD $request_method;
        fastcgi_param SCRIPT_FILENAME /var/www/public/index.php;
        fastcgi_param DOCUMENT_ROOT /var/www/public;
        fastcgi_param HTTPS $fastcgi_param_https;

        # Prevents URIs that include the front controller. This will 404:
        # http://domain.tld/index.php/some-path
        # Remove the internal directive to allow URIs like this
        internal;
    }

This doesn't work though. Since try_files is not allowed inside an if statement ([emerg] 1#1: "try_files" directive is not allowed here).

you can use the error_page directive to handle the OPTIONS requests for paths for which a file exists in the file system.

location / {
    # Try to serve file directly, fallback to index.php if file does not exist
    try_files $uri /index.php$is_args$args;

    # Handle OPTIONS requests for paths for which a file exists in the file system
    if ($request_method = 'OPTIONS') {
        if (-f $request_filename) {
            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
            add_header 'Access-Control-Max-Age' 1728000;
            add_header 'Content-Type' 'text/plain; charset=utf-8';
            add_header 'Content-Length' 0;
            return 204;
        }
    }
}

# Pass all other requests to PHP-FPM
location ~ \.php$ {
    fastcgi_pass unix:/run/php/php7.4-fpm.sock;
    fastcgi_index index.php;
    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}

the try_files directive is used to serve files directly, with fallback to index.php if the file does not exist. The if block checks if the request method is OPTIONS and if a file exists at the requested path. If both conditions are true, NGINX will add the necessary CORS headers to the response and return a 204 No Content status code.