I think one way is ok. to send data from OpenCart to FA and use FA for accounting and tax reports.
Is any link for the ebooks  for OpenCart v2.x? I think if send clients, items and invoices with sold items it will be ok
It will be nice FA to have web shop to be more atractive

Is any opencart 3 FA 2.4.9  integration? Is old one working? Can you give download link?