Desktop must-haves: Choosing our own image hosting providers, and being able to configure our own nostr.json
Discussion
You can change your NIP05 from the Edit Profile view if you want to change it.
nostr.json is server-specific and needs to be changed with access writes to the server. What you mean by configurable?
nostr.json is where NIP05 verification is done. I don't know that changing the file itself is even a good idea, perhaps it could be changed in some instance-specific configuration file?