While I agree more configurable webhooks would be awesome, the current Slack webhook support works fine with Apprise API, which I will wholeheartedly suggest regardless as a “notification broker” to get consistent notification routing across different apps (not just TrueNAS).
In TrueNAS, I use notification type Slack with a webhook URL like the following:
https://apprise.example.com/notify/APPRISE_KEY?tag=truenas&title=Alert%20for%20HOSTNAME&:text=body
The :text=body param tells Apprise how to parse the Slack webhook payload.
In Apprise, I dispatch notifications to both email and Discord as follows:
urls:
- discord://TrueNAS@DISCORD_ID/DISCORD_TOKEN:
- tag: truenas
- mailtos://SMTP_USER:SMTP_PASS@SMTP_HOST?mode=ssl&to=webmaster@example.com:
- from: TrueNAS <apprise-noreply@example.com>
tag: truenas
Or use any of the other approximately one billion notification backends Apprise supports.
(Not affiliated with Apprise in any way, just find it really useful in my homelab.)