How to circumvent "apt-key output should not be parsed"?

From apt-key sources, you can set APT_KEY_DONT_WARN_ON_DANGEROUS_USAGE to a non-empty value to disable this warning.

You can also use "grep -q" so you don't need to redirect stdout in /dev/null either.

I'm modifying the command itself to use batch mode so it will not complain about stdout. For the bigger apt-key this will work, quickly tested on debian:

sed -i "s%{GPG_EXE}\")' --%{GPG_EXE}\")' --batch --%g" /usr/bin/apt-key

While for the smaller apt-key this could work (untested as I can't recall exactly where I have seen this simpler variant):

sed -i 's%GPG_CMD="gpg %GPG_CMD="gpg --batch %g' /usr/bin/apt-key

You need privileges to write to the /usr/bin/apt-key so either run as root or use sudo

To reply to the part "am I getting this fundamentally wrong":

I believe that possibly yes. That's the reason of the warning.

You don't write what you do exactly, but one thing to realize is:

The keys do have some expiration so after some time the fingerprint in your script will become obsolete and possibly it will not behave as expected anymore.

Once the repository and its key is installed so the system somehow upgrades the key automatically but when the repository is initially added so an up-to-date key has to be provided. In my scripts automating the installation I do not test whether a key was already added but I test whether the repository was already added. If the repository was not added, so I add it together with an up-to-date key that I download always from its URL.