The second video with GSX installed is exactly it! That’s what I’m seeing as well.
What I did to try to prevent the issue is also exclude GSX Pro and all of its folders even the ones in AppData from Windows Security. Try excluding all your MSFS related processes, programs and folders and let me know if that improves anything? I did also read that for some reason the heuristics in Windows Defender will sometimes still scan subfolders, despite having the parent folder already excluded. I personally went ahead and excluded some critical sub folders as well and noticed a slight improvement in the stutter. It’s not a complete fix though it seems, but some food for thought.
Gotta love how this sim makes us all tech pros in some ways…