Should I start my initial sync with QB online by uploading a QBI file of my account history that I downloaded from my bank, THEN sync my accounts?
Welcome to the QuickBooks Community and thanks for being a part of the QuickBooks family! It sounds like you're getting started in QuickBooks Online, and I'm excited for you to dive in and get to know the features, such as the bank feeds, which I can see you've already got your sights set on. I can walk you through uploading files from your bank to QuickBooks Online and the best way to go about it.
When you connect a bank account to QuickBooks Online's bank feeds, the program typically pulls 90 days worth of data into the feed for you to sort through. Some banks offer dates farther back than that and others offer a narrower time frame. It's really up to the bank and the configuration for the initial setup.
With that in mind, it's worth connecting the bank feed first so you can see exactly how far back the dates go. That way, you can avoid situations where you come across duplicate transactions from overlap between what you've uploaded manually and what's available for automatic sync. Here's the article to start you off with syncing your bank feed to QuickBooks Online: Connect bank and credit card accounts to QuickBooks Online
When you have transactions you want to manually upload, you can follow the process outlined here: Manually upload transactions into QuickBooks Online. Please note the accepted file types for uploading bank feed transactions into QuickBooks Online: the article mentions QBO and CSV, and the program's upload page also highlights QFX, OFX, and TXT as accepted file types.
Since you have a QBI file, you'll need to explore another option by returning to your bank's website to see what else they have to offer for download. QBI files typically are temporary backup files created while you're working in QuickBooks Desktop. If that's the kind of file you have and you mean to say you want to migrate data from QuickBooks Desktop to QuickBooks Online, that changes things.
When bringing over data from another software, you'll want to do that first because it overwrites anything else entered into the books. So in this case, convert a Desktop company file to QuickBooks Online, then sync any bank feeds. Here are a couple of articles that can help with the conversion process, including what file types you'll want to work with to do so.