But you still need the full blockchain to sign and send a transaction securely, that's why it would be nice to have a really lightweight sending capable client that communicates with a trusted "receive and notify" blockchain to sign the outgoing transactions. Is that what bitcoin-tx is supposed to be?
You don't need to have the full blockchain in order to sign a transaction. In fact, you don't need the blockchain at all. That is how offline wallets work.
What you pretty much described is the principle behind SPV wallets.
bitcoin-tx is not an SPV wallet. It is simply a command line utility for transactions.