It still sounds to me like something's up with the disk. Can't think of any solutions to suggest but I would run a SMART health check on it:
sudo apt install smartmontools
sudo smartctl -a /dev/sda
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.
/dev/nvme0 is probably your SSD. But if it passed you probably have nothing to worry about