I'm suggesting that you can cache both at the data layer and "above the app". I was just highlighting one particularly powerful patterns available if you're having trouble scaling (like my parent said).

> making sure, that when content changed, to invalidate it properly on the edge is not trivial

The trivial way to do this is simply not to invalidate. Include a version in the cache key. When content changes, the version increases, and clients get new versions. (I think this is sometimes called lazy cache invalidation). It works well with LRU caches. It's still a hit for the top-level keys, but you avoid all the heavy data load and rendering.

Granted, that doesn't solve every case, but I'm not sure it's fair to say that it'll take more custom code when the initial approach took "months building custom caching" that never really worked. Also, I think you'll find your origin under less load due to having fewer variants.