iMac slows/stalls after being idle for >5 minutes

My iMac still runs great and has done for a long time – mostly.

One problem is that if I leave the device unattended (not in sleep mode) for anything longer than 5 minutes, when I return to use it, the performance is significantly affected. I get beach balls, windows open extremely slowly, and the menu bar becomes completely unresponsive. Sometimes, this clears up on its own, but in most instances, the only way to get back to normal operation is a restart (which I can sometimes only do manually, because the menu bar is unresponsive). Sometimes, if the device is in sleep, it will not open again, only giving a black screen – when this happens, it crashes (see error report below).

I have checked the Activity Monitor when this happens and there is no significant drain on any resources. I do not use my iMac for intense tasks. This has been a problem for over a year now.

I thought that it was to do with my old fusion drive, so I have now been booting from an external SSD. I no longer use the internal drive. I have also maxed up my RAM to 32GB - both of these changes have made my computer run extremely well, but I am still confronted with the same problem of the device starting to hang and slow down, but only if left idle for about 5-10 minutes. If I use the device without interruption for hours at a time, then I never get any problems. I have performed OS reinstalls and followed all usual advice. I have also reset the SMC, PRAM etc. I cannot work out for the life of me what the cause of the problem might be, does anyone have any advice?

I have also attached an EtreCheck report.