The authorized_keys has a command="..." option that restricts a key to a single command. Is there a way to restrict a key to multiple commands? E.g. by having a regex there, or by editing some other configuration file? //

You can have only one command per key, because the command is “forced”.

But you can use a wrapper script. The called command gets the original command line as environment variable $SSH_ORIGINAL_COMMAND, which it can evaluate.