Is this usable for my entire site?

I would add a custom CacheContext extending the VaryCacheContextInterface, and provide all the wanted Vary headers through the `getVaryHeaders` method.

If you need Drupal to handle variation, you can also compute all those headers in the `getContext` method.

I would do a custom CacheContext per Vary header though.

You could try to "decorate" the existing CacheContexts (like language, cookie, and so on) and provide a "VaryCacheContextInterface" decorator.

You need to create a custom module in any case.

I hope this helps