Some of that just sounds like incompetent design, like how the designer didn't consider the UX of reverting to its previous DND schedule when your temporary DND mode turns off.

Though, perhaps you explained it poorly, what's wrong with it turning off DND mode when your 1hr DND timer expires? What else is the point of the timer?

Does that smartwatch not let you toggle DND off until you decide to toggle it back on? Is there no silent mode?

I keep my iPhone on silent-mode almost permanently. Maybe it's time to try any of the other brands with this elementary feature?

But to answer your question, people generally want to be disturbed by notifications. Just consider how many people don't keep their phone in silent mode. I don't think it's the ideal way to live, but people love running over to their phone to see if it's a new WhatsApp message that cause the ping. Certainly my girlfriend and roommates and all the people I hear day to day who have notification sounds turned on when there's a physical switch on the device to turn them off.

Still, a silent-mode switch already gives you your dream of "I want to opt-in to being disturbed". Apparently every smartwatch except your Xiaomi watch has it. Time to switch.