So theres 2 things I can think to do.
The first is rather superficial …
We can shorten that wait period to, say, 10 seconds.
printf '[Manager]\nDefaultTimeoutStopSec=10s' | sudo tee -a /etc/systemd/system.conf.d/faster-shutdown.conf
(reboot)
The second would be to add a service file to run some commands before shutdown:
/etc/systemd/system/before-shutdown.service
[Unit]
Description=Do stuff before shutdown
[Service]
Type=oneshot
RemainAfterExit=true
ExecStop=/bin/systemctl stop nmb.service
[Install]
WantedBy=multi-user.target
(or however you are shutting down nmb and/or other stuff you want to put there)
Then reload and enable it with
systemctl daemon-reload && systemctl enable before-shutdown.service
(reboot)
Of course you arent limited to one or the other … you can do both.