Check out the article for your 1st question → Build an eCommerce App With Temporal, Part 1: Getting Started | Temporal documentation
I’m no expert but your workflowId can probably be user-collectionId. It is no problem to have 500k workflow, or even 10MM workflows being active at the same time.
Check this guidance to tailor how you’ll do your polling to determine when the bank collection is done → What is the best practice for a polling activity? - #2 by maxim