I'm concerned about your internal SSD since there was a Kernel Panic with the SSD which is usually associated with an SSD that is beginning to fail "nvme: Fatal Error Occurred". That type of SSD failure usually occurs when first powering on the computer & when waking from sleep. Every case where I've personally seen this Kernel Panic has been confirmed to be a bad SSD. There is a slim chance that one of the apps mentioned by @BDAqua is the cause since DropBox is listed in the Kernel Panic. In my own personal experience the SSD is most likely bad.
It is also very interesting that your external Toshiba hard drive is shown first in the EtreCheck report since I would expect the internal SSD to be the quickest drive to go ready. This supports my case that the SSD is beginning to fail.
Another concern is the other Photoshop crash where it had a memory related error "malloc: Incorrect Checksum for Freed Object". This could indicate a hardware issue, but also possibly a software issue possibly from one of the apps mentioned by @BDAqua.
Your internal hard drive is also a weak point since your iMac is about 7 years old, so there is a good chance the internal hard drive portion of the Fusion Drive is beginning to wear out or even fail. You can run the third party app DriveDx (free trial period) to check the health of the internal hard drive. I doubt it will find anything wrong with the internal SSD since that type of failure is related to the SSD's controller which has no health monitoring. Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.
You can also run the Apple Diagnostics to see if any hardware issues are detected. The diagnostics don't usually detect problems, but it never hurts to check. I'm thinking more of looking for possible memory issues than a bad drive with this suggestion.
There is a chance one of your connected external devices could also be causing a problem.
I hope you have a good backup of your system since trying to recover data from a broken Fusion Drive is not easy & will likely result in a huge mess for you. I also hope you have backups of all your external drives as well, including any data in the cloud.
