Check out afew, it helps handle that kind of thing.

https://github.com/teythoon/afew

But personally I just use the post-new hook and use something like this
after my initial tagging:

notmuch tag +sent -inbox -unread -- '(' from:email@test.com ... a long list of all the emails I
send from ')' and not tag:sent and not '(' to:me@example.com ... a list of emails I sometimes
send myself things to ')'

On Apr 01 at 10:04 AM, Laurence Rochfort <laurence.rochfort@gmail.com>
said thus:
> [ text/plain ]
> Hello,
>
> I'm trying notmuch via the emacs interface for the first time today.
>
> I've noticed that messages that I've sent are tagged with "inbox", rather
> than "sent" as I'd expect.
>
> I've configured the notmuch-fcc-dirs as follows in my init.el.
>
> (setq notmuch-fcc-dirs "Sent")
>
> Should notmuch emacs client mark sent messages with a sent tag? If not how
> can I do so and stop them appearing in my inbox search?
>
> Cheers,
> Laurence.
> [ text/plain ]
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