Automation for Public Accountant

I’m trying to help my relative automate their accounting firm’s tedious Excel-based workflow and need advice. They download raw Excel files (sales/purchases) monthly, sort them by company name, and transfer data into a workbook with Incoming (purchases), Outgoing (sales), and Client List sheets. The Client List auto-fills details using tax codes. They manually move sorted data to the right sheet, update cell references (e.g., pointing to row 3 in Incoming), and verify totals, splitting dual tax rates (e.g., 30,000 invoice with 20,000 at 10% VAT, 10,000 at 1%). Finally, they paste values.

I’m using n8n to automate this, but I’m stuck: sorted data lands in the wrong rows (e.g., row 5,000 instead of 23), and accessing Excel workbook/sheet IDs programmatically is tough. I even couldn't find the workbook and sheet IDs. Google Sheets seems easier, but they’re stuck with Excel. I’m also considering AI (semantic) to handle messy client data but prefer simple coding to keep costs low. Any tips on automating this better? I’m limited to automating existing processes without changing their setup.