iMac 5K random shutdowns and suspecting failing PSU during photo editing

This is starting to sound more like an over-heating GPU, given your usage and when it goes belly-up.

Basically the GPU over-heats and then the system shuts down to prevent a total meltdown.

What I would do, is get and use MacFanControl to monitor the GPU temp. > https://crystalidea.com/macs-fan-control

Set up MFC to monitor the GPU temp. on the menu bar and/or on the main MFC window.

Then start editing a photo and watch for the GPU temp. to spike.

As den.thed requsted please download and run Etrecheck.  The free version is sufficient. Be sure to give it Full Disk Access.

Copy the report as shown in this animated screenshot

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

1) EtreCheckPro is an excellent tool for finding and sorting out problems.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

2) Full Computer Sleep Mode is causing an assortment of issues including crashing during sleep.

As a test:

Turn On "Prevent automatic sleeping when display is off"

Set "Turn display off when inactive" to 5 or 10 minutes

Leave the iMac On and only allow the display to turn Off on timer

Do not manually Sleep from the Apple Drop Down menu

Shut Down the iMac when away for a day or more

I don't see any of the usual suspects in the report.

Do what den.thed recommended and don't put the Mac to sleep. Just use the Screensaver and after x minutes darken the screen. For long periods of inactivity or overnight shut the iMac down.

Where did you get the additional RAM modules. If not from Apple, Crucial.com or OWC (MacSales.com) then take the added modules out and run for a while with the original modules to see if the problem persists. If they are from one of those three make sure they are firmly seated in their sockets.

In addition to testing the Energy and Lock Screen settings.

Test with the Kingston Multi-Reader Hub unplugged for a day or so.

Not good, sorry to hear that.

The Apple Store will want to replace the Logic Board, PSU, etc. in order to guaranty and that's going to cost.

Personally I would put my money into a new Silicon Mac and repurpose that iMac for more mundane tasks.

I use the Sensor-based control on the sensor with highest temp. readout.

Just keep in mind, that some areas heat up slowly and others rapidly spike under a load.

I don't see anything own the new report that might indicate what the root problem is. Time to make an appointment with the nearest   Apple Genius Bar for a more definitive, free diagnosis.