I actually know about Openwrt and it's option 6 box and I believe your second example is called tagging.
For over a year now I have been disabling Openwrt's DHCP server and using 2 Pihole"s, each running DHCP and DNS.
I just got a new Samsung S8 phone before Christmas and noticed in my logs that even though I had setup option6 on the DHCP server's to point to my PiHoles it would bypass them and go strait to Google's DNS servers every other device would use what I told it to.
This is a new feature that newer Android devices have started (for me) since Android 8 Oreo and the only cure I have found is to block Google's IPv4 and IPv6 DNS servers, This forces my phone to use my server and the iptable rules help which I might change over the weekend for a more approved method.
More and more devices in the future are going to hard code in their own preferred DNS servers.
I applaud Google for making the internet a bit easier for the masses but I wish they included a way to disable it for fringe cases like me, I have no idea what corporation's do who have strict rules about logging and what sites people are aloud to visit.
About pihole if you have never heard about it.