Erstmal Leo fragen ...

he held all the strings (in his hand)

geht hier z.B.

Edit: pulling them is going a step further than merely holding them, IMO.

Habe schon Leo gefragt Corduli, aber klingt das nicht etwas seltsam?

Although he wasn't actually aware of it, during production it was him who was pulling all the strings.

#2: not only that; pull the strings has a negative touch in English, I'd say - remember "Puppet on a String"? And just holding them doesn't say much, in German it means more, I think.

"call the shots" maybe, if that's not too slangy.

or "...he was the one who actually coordinated everything during production." ?

Danke für die Nuancen. Wieder was gelernt.