Just curious, but if they sent no treasure fleet for fear of capture,
Would they have sent any ships?
Percentage wise, I would guess that most ships returning to Europe contained treasure,
Not much benefit in sending an empty ship back, as stated in your post, "he was able to keep the treasure fleet in port"
I am sure they had beancounters way back then, and the beancounters would have advised againstr sending empty vessels back, granted, some may have, but I am guessing the majority made the trip laden with valuables,
All in my uneducated estimation, :)