You'd think there were plenty of such apps to quickly:
* set a timer for a 20 or 30-minute nap
* at the same time activate Do Not Disturb mode
* and have an alarm to wake you when the time expires
* then disable DND mode.
Well, many don't mention activating Do Not Disturb (and some I tested certainly did not activate it), and whilst the built-in DND mode does have a timer option, it has no alarm to wake you. Nap Time Alarm by Hil Liao is an open source app that does the job, but it is a bit clunky, and the dismiss alarm does not pop up into the foreground to quickly turn it off.
I tried the Samsung Routines app, but it is finicky in that you can't add DND a second time to disable it (and the reverse option did not reverse DND). I did actually manage to manually set up a task in Tasker that actually does the job exactly as I wanted it.
So I found 'Power Nap one touch - Simple h' by Trecin. Although it was last updated in May 2019 it seems to work perfectly for me on Android 13. On startup, it has a bunch of standard nap times where you press the button, and it just starts the timer (with DND activated, or you can toggle DND to stay off), and pops up the alarm at the end of the period. You can also set the alarm volume to gradually increase (over a period of time) as well as a snooze option.
See Power Nap one touch - Simple h - Apps on Google Play
Power nap alarm with one touch. Nap time clock with alarm sounds - free