Hey @jack.bates! Sorry to get to this so late. (Thanks, @twalane, for alerting me.)
It looks like you are making progress on this but I wanted to make sure you saw this import tool. It allows you to build a custom column mapping for your CSVs without requiring scripts. Essentially, once your mapping is constructed, you can just import the bank CSV as a new sheet, have the helper spreadsheet map the columns to match your Transactions sheet, then copy and paste the organized data as values into your Transactions sheet.
If you are already onto a more robust solution, that’s fine. Just wanted to make sure you saw this.
Randy