I am still not sure if I understand what you need.
If you have a Smart Object, which occurs multiple times in your PSD, you can double-click it, put all your screenshots inside it and save it. Then you can simply switch the visibility of layers inside a smart object.
Using Photopea would require running it in a browser window, it would be hard to connect it with your terminal.
I want to show various screens on the same template. Currently, I take the screenshot and replace the images one by one.
Ideally, I would like to issue a command from my terminal to use the screenshots (which I generate from another tool) in a folder and pop them into the smartobject and export the output to an output folder.