[Google Pixel Watch] breaddisease:
Hi all, after a few days of failed rooting attempts, I finally got root again! I ended up patching the boot.img on the watch using the Magisk command-line scripts extracted from the APK. Below are the commands I used:

Enable developer options on the watch by going to Settings > System > About > Versions > Tap Build number until you unlock developer mode

While here, note down the Build Number. It will look like RWD9.XXXXXX.XXX.XX.

Download the appropriate (LTE or Bluetooth/WIFI) firmware at the link below on a computer, making sure to download the version that matches the build number from Step 2.

https://developers.google.com/android/images-watch

Verify the checksum of downloaded firmware using

sha256sum name-of-firmware.zip

Install the latest version of ADB and Fastboot tools on a computer

https://developer.android.com/tools/releases/platform-tools

Go into Developer options on the watch and enable ADB debugging

Connect watch to computer using USB/wireless debugging and allow permanent ADB debugging access on the watch

Verify your access works by running

adb devices

Unzip the downloaded firmware zip, then unzip the image-rXX zip inside the original zip. Then, transfer the boot.img file to the watch using

adb push boot.img /data/local/tmp/

Download the latest version of the Magisk APK from the Github link below and and change it to a .zip file

mv magisk.apk magisk.zip

https://github.com/topjohnwu/Magisk/releases

Push the name-of-magisk.zip file to the watch

adb push magisk.zip /data/local/tmp/

Enter an adb shell

adb shell

Change directories to /data/local/tmp/

cd /data/local/tmp/

Unzip the magisk.zip file

unzip magisk.zip

Copy the shared object files for the correct architecture to the assets folder

mv lib/armeabi-v7a/* assets/

Rename the relevant files so the boot_patch.sh script can find them

mv libmagiskboot.so magiskboot; mv libmagisk32.so magisk32; mv libmagiskinit.so magiskinit; mv libmagiskpolicy.so magiskpolicy; mv libbusybox.so busybox

Make all the files executable

chmod +x *

Push the stock boot.img file of your watch's version to the watch

adb push boot.img /data/local/tmp/assets/

Run the boot_patch.sh script with the boot.img

./boot_patch.sh boot.img

Pull the patched boot.img to your computer

adb pull /data/local/tmp/assets/new-boot.img .

Reboot the watch into the bootloader

adb reboot bootloader

Verify the device shows up by using

fastboot devices

If your bootloader isn't unlocked yet, unlock the bootloader of the watch using

fastboot flashing unlock

Confirm the message on the watch to unlock and wipe the device

Flash the boot image using fastboot

fastboot flash boot new-boot.img

Start the watch from the fastboot menu

Once booted, reinstall the Magisk app if you wiped your device

adb install name-of-magisk.apk

To prevent the broken Superuser request popup where the grant button is off the screen, open the Magisk app, go to settings, and scroll down to Automatic Response. Change the setting from "Prompt" to "Grant" by swiping up.

Then, verify you have root by running su in an adb shell

su

Hopefully this works for others as well! I will update the main post after I get back from hardwear.io. Thanks!