I am in the same scenario.  I have my chart of accounts created and am going back to the beginning of this year.  What I have done (and it has worked well for me so far) is downloaded all my statements by month as .qbo files and imported them.  I can categorize each transaction and make sure it is mapped properly.  Yes, it is time consuming, but I know it will be correct!  I tried linking all my accounts initially, not realizing the automation only went back 90 days.  Uploading prior to that 90 period turned out to be a NIGHTMARE for all my balances......if you don't want to do it personally, you can always hire a QB consultant to get you started and get you familiar with this platform.