Yes, canary domain is fine - at least while it still works. I don't want to have to configure DNS for 50 different applications on 20 different machines. My OS does DNS, and my DHCP server doles out the DNS server I want my machines to use.
> The use of this domain is specified by Mozilla, as a limited-time measure until a method for signaling the presence of DNS-based content filtering is defined and adopted by an Internet standards body.
Presumably that'd be the IETF, and I have no idea which committee would be working on this, if there's a draft RFC in the works, etc. But it's clear the expectation is some sort of standard mechanism will eventually replace the canary domain that yields the same functionality but in a less hacky way.
There was [1] which would've allowed websites to specify custom DoH resolvers for their subdomains, but it expired some time ago. The same authors are now working on [2] instead, which AFAICT is basically for the same thing.
There's also [3] which is a way for the DHCP server to also provide DoT and DoH servers to the LAN (in addition to the usual DNS servers).