Another vote for track standing. You anticipate the stopping and slowly get into a position where you can balance without moving. There is nothing hurried so if you want to unclip - you do so. It is all about planning. I have days where I don't touch the ground except for at home and at work. Long red lights are just another challenge and it gives you something to do while waiting - sort of like fidgeting.

As for SPD - I prefer then over flats but I ride both. I don't find there is much of an adjustment as I rarely take my feet off the pedals. Maybe I unnecessarily twist my ankle out on flats since I am so used to it? I also make sure my shoes are walkable. I couldn't care about how stiff a shoe is - I only care how it feel s when walking. I often wear my clipless shoes all day at work without issue.