Is there a way to copy all tags from one FLAC file to another seamlessly using a command-line tool?
If you only want the text part of metadata, you should use metaflac with the
--no-utf8-convert option on export and on import, otherwise characters that are not in your shell's characters set get mangled (e.g. for me Japanese metadata did get mangled without it):
metaflac --no-uft8-convert --export-tags-to=- src.flac | metaflac --import-tags-from=- --remove-all-tags dest.flac
no idea why utf8 is not the default.
This is not complete. If you want to move images as well, a small python script leveraging mutagen is the best way to copy all metadata.