I understand this is something you need for your business. It'd be pleasant to identify the sequence number for credit memos and invoices, however, this option is currently not available.
Letting us know what works best for you will also let our Product Developers know what they need to consider in future updates. Rest assured that I'll relay this message to our team in concern here on my end. We'll take this chance to fully support our customers in a well-timed and efficient manner in the future.
Meanwhile, I'm adding here some helpful resources for QBO users. It provides the articles, tutorials, blogs, and overview of the product.