My devices get their dns servers via DHCP. So I enter it only once on the DHCP server.

If you’re running so many devices, you probably want to use your own DNS server too (for internal name resolution), so you can also set the upstream just there.

I feel your pain but if you really have so many machines you probably automate their configuration.

Or, setup everything to use a resolver on your router, configure your router once. So when you want to change in the future, only one place.

That's what things like Ansible and VM base images are for.