Why does my iMac crash when I leave it for a few hours?

IMHO, it does not sound like the problem was fixed. It sounds like you were given a workaround that has become problematic.

I think we need to have a look at what all is running on your Mac.

I advise that you run an EtreCheck evaluation of your Mac. EtreCheck is a safe and highly regarded utility from a trusted developer and respected ASC contributor. The diagnostic app gathers important info about your hardware and software and reports it back to you. The EtreCheck report will not include any personal info. It simply gathers specifics about hardware and performance and any installed software that might be in conflict with the OS.

Please navigate to EtreCheck.com and download the free version. Be sure to enable full disk access when you install the app. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. To see how, please click >  How to use Add Text when posting… EtreCheck Report.

We’ll use your EtreCheck report to look for the things that may be causing your problem and advise how to correct them.

Please post the entire Kernel Panic here using the "Additional Text" icon which looks like a piece of paper. Kernel Panics are located in "/Library/Logs/DiagnosticReports" folder. If you don't see the panic report there, then check the "Retired" folder and any other folder within it for a recent file with "kernel" or "panic" in the name. Post several if available.

FYI, I just saw a very similar Kernel Panic from a co-worker's Mac, but that report had everything packed in without line breaks making it impossible to parse, but it did have "initproc exited -- exited reason namespace", but the rest of the log was impossible to read due to the formatting unlike your screenshot. For the coworker I am fairly certain it is a hardware issue as their model laptop is known to have Logic Board issues (MBPro 16" 2019). I still have a couple things to try before confirming though.

It seems like the tech discovered a failing internal drive in your iMac if they had you use an external boot drive. But a failing internal drive can cause issues even when booting to external media if the internal drive failure has become severe.

A sleep to wake transition can cause delays in processing since the system is likely trying to communicate with the failing internal drive.

Or you may have some other hardware issue.

Try running the Apple Diagnostics to see if any hardware issues are detected.

If a failing internal drive is causing your problem, then the only way to resolve the issue would be to open the iMac and replace the internal drive. Just disconnecting the internal Hard Drive is not sufficient since the iMac uses the drive's temperature sensor. Without the internal hard drive, the fan(s) would run at high speed all the time unless an OWC temperature sensor adapter is installed.