I made account just to chime in on this thread.
1. I'm not aware of any HDR screen manufacturer who has terms like 'sustained' and 'peak' brightens associated with SDR specifications of the screen.
2. All Apple HDR screens are branded as XDR, having XDR Pro display as flagship while other products are catching up, one by one (iPad Pro, now MacBookPro). So by pure logic and what is stated in all materials, specs of XDR displays are inherited from most expensive model, hopefully.
3. SDR and HDR are terms reserved for video and photo content, media creation and consumption. I never ever heard of HDR emails or word processing or coding before reading this thread.
4. I understand that someone wants to work outside and ramp up screen up to 1000 nits. And that could be possible soon, but you barking at the wrong tree here. In order for that to happen, software companies needs to enable PQ curve in their UI design, rethink interface for high contrast scenario and thats it. But let's blame Apple for actually making that doable with no actual software to support HDR interface.
5. Not sure how smart would be to have HDR interface: shorter screen life, battery life, burn ins...
I'm surprised with these complaints, expectations and with general lack of knowledge about matter discussed.