NGINX Set Reverse Proxy Header based on the URL

I have set up a reverse proxy for the static content on the site. Everything works, the only issue is there are a few pages that the static content is not at that location. I am trying to do something where I ONLY proxy that content for some of the pages, or easier in reverse, proxy the content for all pages but a few.

Current Proxy

location /sites/default/files/ {    
    proxy_set_header Host url.com;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Host $host;
    proxy_pass https://url.com;
}

But I need some way to do the following (I know this syntax is 100% wrong, just making up to give an idea what I am trying to do)

location /sites/default/files/ {
    if (request_url != '/cart' || request_url != 'checkout') {
        proxy_set_header Host url.com;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Host $host;
        proxy_pass https://url.com;
    }
}

After hours of testing and Googling I have a fix for the solution. There were other similar ones out there but none that were exactly what I was trying to do. Here is what I ended up doing to get that to work.

First I created a variable map that conditionally sets the value based on the $http_referer value that is within NGINX

map $http_referer $resources_location {
    default     "url.com";
    "~*/page2"  "url2.com";
    "~*/page3"  "url2.com";
}

This takes the variable $http_referer and sets the variable $resources_location based on the value of the $http_referer.

the default is the main url, and then I have regex expressions to determine of the page has /page2, /page3 in it.

The one catch I assume is https://url.com/page2 would trigger it, so would https://url.com/sub/page2. I do not have the need to worry about that, but one might so I would test also.

Then I use the new variable that is set in the proxy

location /sites/default/files/ {        
    proxy_set_header Host $resources_location;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Host $host;
    proxy_pass https://${resources_location};
}  

Im a newbie to NGINX, so if there is something that can be done better, let me know!!