Finished Designing Data-Intensive Applications. What next?
Papers, in small doses: the Dynamo paper for the tradeoffs, Raft for consensus you can actually follow, and the Google SRE book for how it fails in practice. Another textbook at this point adds vocabulary rather than understanding.
I read and forget. What note system survives?
Notes that answer questions you had, not summaries of chapters. A chapter summary is a worse version of the book; a note that says "this is why our retry storm happened" is retrievable years later because it is attached to a real problem.
Anything that changes this at ten times the scale?
At ten times the size the bottleneck moves from the operation itself to what it competes with: memory, locks and the people who have to run it. The approach stays, the batching gets smaller and the schedule matters more.