Which is defined as Signature separator in RFC.
This helps clients like Thunderbird to remove those on replies.

"Dear..." isn't a signature at all.

What you want then is a template, not a signature.
see bug 0004320

Hey @Christian Mack, what about the dashes "--" that do get added before the signature. How do you actually disable them?

First those are two minus signs followed by a space, no dashes.

Second you don't want to disable them.
They say "here starts my signature", then follows the signature.

As I said before, if you do not want a signature, you don't have to use one.
But if you do use one, you do use one.

BTW:
Easiest workaround for not having templates in SOGo is to copy some template into Drafts folder and use them from there.