I wonder how you deal with restarting changefeeds? The last time I checked you'd have go through every document again after losing the connection to rethinkdb or restarting the server.
We use changefeeds more or less of a queue/pipeline and don't care too much about the initial state. When the changefeeds are created we specifically don't pass the includeInitial argument [0] so we only get a stream of newly modified/created documents.
In a slightly different use case than what OP is describing, we keep track of createdAt and updatedAt in Rethink, order by those, and pick off from max(createdAt) in destination in order to fake restarting the feeds.
[0]: https://rethinkdb.com/docs/changefeeds/javascript/#including...