That's a matter of habit. The trigger before was space. "I am in the office - work mode on". In WFH the trigger has to be time. We have a hard 9AM start policy

That's a matter of habit. The trigger before was space. "I am in the office - work mode on". In WFH the trigger has to be time. We have a hard 9AM start policy, with 12 to 1PM lunch. That puts everyone on the same page and gets rid of the ambiguous "I wonder if anyone else is looking for me right now, and should I be working". You should absolutely know. If people don't respect it, it's their problem, but you should not be responding to people at 7AM or 9PM unless the sky is falling. No different from how it was before. This lack of boundaries is what really exhausts people. Also, create a separate account on your computer for work. You should do that anyway to avoid screen share mishaps (no one wants to see your personal bookmarks and history). At end of day, log out, killing slack notifications and email, and log into your work-free personal account. You are done for the day.