@James Sam - A Google-Drive-like experience is not currently available.
A possible workaround would depend on your use case.
If you were concerned with accidentally making changes, you could select all using Ctrl/Cmd+A and the lock using Ctrl/Cmd+L. Then, when you wanted to get back to edit mode, you would select at least one of the objects you previously locked and unlock, again using Ctrl/Cmd+A. The benefit of this the approach vs using the Unlock all feature is that you may have had objects locked prior to your mock “view only” mode and want to maintain those locks - using Ctrl/Cmd+A only selects unlocked objects.
If you just want to experience the board in “view-only” mode, you could set its Share settings to Anyone with the link → Can view and open the board in a private/incognito window. Note: This may not be possible if you are on the Enterprise Plan and the Sharing Policy is configured to disallow public link sharing.
Exactly what I wanted to know. Thanks so much!