The problem is that you are saving a REFERENCE to the object (object path), not the widget itself. Your save file should store the DATA from the widget (texts, image links, etc.) that you can use to “recreate” the original widget.
Serialization will help with this in the most automated way, but you will need minimal knowledge of C++.
But in principle you can limit yourself to blueprints only, you will just need to create many variables for different types of data…
Is there any point in trying to save not the widgets, but the card indices in the card database, so that the deck can be reconstructed using these indices?
The saving system in my projects was built on this guide. You can save not only actors, but any objects, only the function of creating objects will differ, and perhaps the moment when you loading the data.