Moisture collecting underneath condo terrace deck boards
You should replace the boards with composite decking.
I don't know what the old boards were or how old they were.  A roof top deck that covers a membrane is always going to have moisture.
The deck structure looks to be 12&quot; oc so you should be good to use composite.
If you flood the membrane with water chances are there are low spots that pool the water for portions of the year.  It will never be perfectly dry in there.  You don't want larger gaps in between your boards unless you want to loose coins, keys, etc in between the boards.  Larger gaps would probably help somewhat with ventilation but I doubt it would be a magic fix.