Are you concerned about that for performance reasons or for ease-of-use reasons? To address the latter, I made a separate transaction format for Biff that allows merging (it's similar to Firebase's transaction format). So e.g.

  {[:users #uuid "some-user-uuid"] {:db/merge true
                                    :username "foo"}}

gets converted to

  [[:crux.tx/put (merge (crux/entity db #uuid "some-user-uuid")
                        {:username "foo"})]]

There is a race condition though; if another tx updated that document after the crux/entity call but before `:username "foo"` was written, then that tx would get clobbered. I'm planning to add `:crux.tx/match` operations automatically to prevent that from happening. There was talk on #crux in Clojurians slack today of using the newly-added transaction function feature to do merges. Haven't looked into that myself.