If the answer is writes, will each DB have a different preferred node for transactions and does cloud tries to distribute this evenly? or will a single node, at any point in time can be the preferred one for multiple databases?

If it's the latter, sounds like you are better served by creating multiple production stacks to better distribute writes among databases than by increasing the node count for a single production stack (sounds like a nightmare though) or.. instead of increasing to such many nodes, have less nodes but increase their size, ex: to a i3.xlarge

Using Datomic on-prem, I am trying to migrate a :db/ident to a new alias (while keeping the old one for existing code). The docs suggest this is possible: https://docs.datomic.com/on-prem/best-practices.html#use-aliases Unfortunately, the documented approach asserts the new ident and removes the previous one:

[:db/add :old/id :db/ident :new/id]

;; => #:db{:id 10, :ident :db/ident, :valueType :db.type/keyword, :cardinality :db.cardinality/one, :unique :db.unique/identity, :doc "Attribute used to uniquely name an entity."}