On my phone, I have my (google) digital wellbeing app set to enable bedtime mode when plugging in between 23:00 and 7:00. So when I plugin my phone to a charger in that time, it goes into bedtime mode.
Now, I suggest to implement a feature to trigger bedtime mode on the watch, when the phone is plugged in and disable it when unplugged.
Tl,dr: Trigger bedtime mode on watch based on whether the phone is charging or not.