Hey, thanks for the answer.

Not sure if I'm missing something or don't understand the magic behind hard-linking to the users home but with my limited knowledge I see following problems:

As the .cookie file is always created anew on bitcoind start and removed once bitconid is stopped, I don't know if chgrp would set the group permanently for the file. I wouldn't expect it to, as it's always a new file with new content. That's also the reason why I went via the bitcoind.service route to set the group ownership.

And even it it would preserve the group, how can the hard-linking to the other users home extend the file permission from a file it couldn't read in the first place?