This is what I do to sign all my messages:

--8<---------------cut here---------------start------------->8---
(add-hook 'message-setup-hook 'mml-secure-sign-pgpmime)
--8<---------------cut here---------------end--------------->8---

I don't set mml-secure-openpgp-signers nor mml-secure-opengpg-sign-with-sender.

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/