Let's first understand your crashes. The game should not crash in HDR mode even when Mac does not support HDR. Please look inside User Reports in Console for DOS2 crashes and send it to the support team please.
Next stop. 5K iMac definitely supports HDR. Can you please go to System Preferences and tell me whether you see the setting called 'Automatic Brightness Adjustments' and whether it is on.
Launch the game. The fire you see should be VERY bright. Also you didn't calibrate the HDR picture, that's why you're having issues. Go to options HDR settings and adjust it. Generally you should set brightness to max and contrast to min to get 100% clear result, then adjust accordingly.
