I'd be pleased to walk you through how you can import transactions from closed credit card accounts in QuickBooks Self-Employed.
You have the option to manually import your transactions in QBSE even if your credit card accounts are closed. Most banks let you download transactions from their website into a CSV file. If you use spreadsheets to track everything, you can save your work in this format. After that, you can upload this to the system.
To do this, you'll need to go to your bank's website and look for the transactions that need to be imported. You can export it from there and upload it after that. Here's a link that will give you more insights into this process: Learn how to get transactions from before you connected your accounts.
Once done, you can now import it in QBSE by following these steps: