Definitely flatpak related then. Try running one of your flatpak apps from the terminal, and post the output here; might help pinpoint the issue. You can list the ones you have installed with flatpak list, then flatpak run <one of the listed apps, e.g. org.videolan.vlc>.
If you prefer a graphical tool, you can do the same thing with GNOME Disks, which also has options for disk benchmarking.
In the resulting report, the overall health state should be “PASSED”, the “Type” column should show “Pre-fail” and “Old age” values, and the “Media-Wearout-Indicator” should be close to 100. If the overall health state is “FAILED”, then you will want to back up your files immediately and consider getting a new SSD.