My Intel Retina iMac 5k can, with a handy utility, be run at 3840x2160 in HiDPI or up to 7360x4320 (foregoing HiDPI.) M1 Macs can’t. They only go up to 3008 by 1892 in HiDPI, and are missing some desirable scaled resolutions.
Steve Wozniak uses the utiity. I’m hoping we can persuade him to raise this with Apple. His people are at: teamwoz@woz.org .
The utility is QuickRes, but for readers I’d recommend Resolutionator, which is almost as good and is only $3 instead of $15.
The native resolution of the 27" 5K iMacs’ display hardware is 5120x2880 so a setting of 7360x4320 is not only forgoing HiDPI, it’s not actually showing all those pixels. There are tasks for which such a high resolution is useful but I would think it would be hard to use that setting all the time.
The 24" M1 iMacs have a native resolution of 4480x2520, you should be able to choose that by Option-clicking the Scaled radio button in the macOS Display System Preferences. I don’t know how many other resolution choices the System Preferences offer for that iMac’s display.
Both QuickRes and Resolutionator have been updated this summer and say they support M1 iMacs. The M1s have different display hardware than the 27" iMacs so I’d expect some differences in the resolution choices. I wouldn’t expect the choices to be significantly restricted, including the “silly resolutions” as Resolutionator calls ones that exceed the native resolution of a display.
System Preferences’ choices are unfortunately restricted on M1. Even with the 6k display you can’t get higher than 3008x1692 HiDPI.
I’ve posted a summary of what can be done:
https://penhelm.com/data/Mac_extra_high_resolutions.html