Do I need soffit vents and can I add them without taking the roof apart?
The best and easiest solution is unfortunately no longer possible without removing and reinstalling the metal roofing: you vent above the sheathing and below the metal roof. The way you would do this is by installing the metal roof on vertical eave-to-ridge battens, with a metal ridge vent piece at the ridge. This way, all the ventilation is on the outside and you don't have to worry about complicated interactions with your finished attic.