MongoDB Sharding : What should be the Sharding key if we want to find records with more than one field
From what you share the url seems the best candidate for shard key. Has good cardinality, its not monotonically increased  (since is url), does not modified and don't contain null values.

Additionally your reads contain the url will directed to only one shard, which is the best scenario.

We can discuss that further on chat after Xmas, because sharding isn't always easy task.

Antonis