Do I need to make sure that all bank feed transactions have been added to QB desktop before I back up the desktop file to convert to QB online, or can I just back it up as is and then add all the outstanding transactions after the conversion?
When converting or switching from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO), bank and credit card accounts set up for Bank Feeds in QBDT will lose their connections. Also, the bank rules don’t convert. You'll need to reconnect your online banking to continue downloading transactions.
That said, either of the two options you've mentioned above will work. You can choose whichever is more convenient for you.