In process of closing our company, how do I back up data in case we get audited by tax office in next 7 years?

You can NOT back up QBO

The only real option to get all your data for archiving against a future audit is to have an accountant log in and print or export to excel the general journal. (Or invite yourself as the accountant, create the log in, and log in as the accountant so you can use the accountant tools.) Due to QBO's limited data handling capability that will probably have to be done in date segments. Of course that may not help in the event of an IRS audit since they have stated that the actual software data file is what they want, and an excel spreadsheet is not satisfactory.
(Note: simple start does not have a true general journal report at all.)

QBO data handling capability for reporting is not adequate, if there is more than 5K records it times out and reports that you have too much data. You will have to do a journal report in date range segments, like Jan-Feb, Mar-Apr, etc etc.
How many months you can span per report is a guessing game since the report can not deal with more than 5K transactions.