You are correct - that would work to send reminders (notifications). However, we wouldn't be able to prevent people from using their apps. This is definitely something we are considering.
The desktop problem is pretty solvable for programmers. I wrote a simple bash script to add/remove blocked sites to /etc/hosts. I mapped that script to aliases 'focus' and 'distract' that block and unblock sites, respectively (requiring the root password). Finally, I set up a root cron job that runs 'focus' every hour during desired time windows (for me that's morning until late evening on Mon-Sat inclusive).
That system works exceedingly well, but I can't port it to iOS.
Right now I use Lockdown (VPN-based) with a block list and it works okay, except when I want to unlock by disabling the firewall. There's no automatic reenable option making the whole system less effective. I considered writing my own VPN-based system but haven't got around to it. Would love to see Motion tackle it.
It is not trivial to build, run, and operate a nextdns-esque service. The founders, Olivier and Romain, are some of the world's best at what they do, imo.
That said, a client-side DNS re-write solution might be relatively cheaper to implement and might even be a tad-easier than deploying an global DNS resolver with per-user custom configurations and endpoints.