The default notification experience of a pixel watch is roughly akin to strapping the whole internet directly to your body and cranking it. Here's what you need to do disable ALL the notifications.
In the pixel watch app on your phone, under Notifications, deselect all the watch apps and all the phone apps. In the General section disable "auto turn on notifications for new apps". Under the "mute notifications" section, enable all the "mute ..." options in the Watch section.
In the watch itself open the settings (cog icon), Apps & Notifications, Do Not Disturb, enable DND.
In the fitbit app on your phone, click the profile picture icon in the upper left corner of the app. Then click the "Google Pixel Watch" item in the list (surprise, that's a button!), then disable "Reminders to Move". While you're in there make sure that "Main Goal" is set to "Steps". Back out of there and click the "Activity & Wellness" item under the SETTINGS heading, then click "Daily Activity", and set the "Steps Goal" to 999,999 (the highest it will accept. Now never take more than a million steps and you won't be interrupted again.
Nice enough watch I guess but it is extremely one point oh.