Pool Deck Expansion Joint

Clean crack of debris, add backer rod then Deck-o-seal.
How would you handle the fact that most of the pool is fine with that VERY thin spacing, and only that one area is wide. Should I just address that wide area, or try to do the entire thing? I thing that thin area is like the width of a quarter
 
How would you handle the fact that most of the pool is fine with that VERY thin spacing, and only that one area is wide. Should I just address that wide area, or try to do the entire thing? I thing that thin area is like the width of a quarter

That actually is a problem and indicates that your pool and deck have been moving. If the area with thin spacing closes up it will start putting pressure on the pool and possibly crack the bond beam.

You should saw cut a wider expansion joint to give space for movement and fill with flexible mastic.