In fact, using DND mode,
many users wish to watch a movie or listen to music.
Ringer is the only one people need to protect from disturbance. Their
Alarm is the thing that turns DND mode off to wake them up. And so on.
If your phone rings and you don't hear it, but you do see it, and you want to answer and hear your friend, Ringer is the only thing to mute. Your earpiece and mic should be unaffected. This last point might be obvious, but the first two points are also obvious to me.
In short, what you notice is the designed behaviour. If you turn DND on, the user experience is intended for Ringer to only respect DND.
I have found this totally useful because I assign everything to a channel of my choice. I have become accustomed to 3-5 actions in any task, specifically to set the volume of a notification I always want to hear and leave the task, setting it back to whatever the level was before the task ran. This way, I no longer miss reminders I most certainly want every day (like medication), even when I turn my phone down randomly.
Working within the limitations or design of Android is sometimes the way to go. At least when it gets quite annoying to chase such wiring. It took me some time to sort out the logic, but now I find it straightforward. One of the keys for me was indeed placing virtually every custom notifier on Media, and like I said, controlling its volume per task was the way to go that was cleanest rather than sending "what I want" through four output categories.
You mention "one place to temporarily turn off DND."
I literally designed this Tasker Scene, which I show by tapping a shortcut on my home screen.
Mute is essentially a perfect DND, as you describe (in audio terms). My phone visually stays the same; calls appear. It's not quite Android DND via quicksettings.
Then ZzZ is, audio-wise, more like DND, as you have a complaint about (Media still plays).
Day, Outside, and Loud get progressively louder. I made these to change all volume categories with a button, so media, ringer, and alarm would be where I needed them.
"LongTap HERE" does the following: if my Current state is "day", then I change the volume rocker on my phone and make changes, and LongTap HERE will overwrite Day with the settings of the moment.
It's kind of like self-monitored "machine learning."
In very short terms, what you said in the subject "Do Not Disturb ignored by Tasker"...
I describe it as "Android being Android, before even considering Tasker."