Not for everyone IS really dependent on expectations! If you expect something that leaps into the sky in absolutely no wind - sorry, not happening! Hope that didn't burst your bubble, but way too many have bought it expecting something it isn't!! ANY quad requires good footwork to fly! Light wind may require more! You will find that the Zen really shows any flaws in your flying, The suggested LE swap is a very good fix, change it - ASAP!! The stock frame IS rubbery, you never know when to give the next input !! And you need to learn how to baby it around. It isn't the sports car a 1.5 is, rather a big bus, that needs room and time. That's why the longer lines work - more time to let it react!
I've also decided on adding some magic sticks to mine. Been almost 2 seasons now, I like how they restrict the LE, making it even stiffer - to my touch anyway! And I like your selection of lines - I also use either 50# or 90# in close to the same lengths as you! 50' for solo work, 75' or 120' for duets.
I do fly it solo, but I fly mine more than most. It was designed to be a team kite, but the over steer was a problem in team flying. Changing the LE helps, but doesn't eliminate the problem - makes it tolerable!!
As far as handles go - I have 13", 14", and 15" ones - probably use my 15" most of the time. But not so much for inputs as more light wind leverage. Don't know the tech reasons, but the longer handles just seem to lift better. However as the wind comes up, the shorter 13" will feel better! Or just swap to a STD!! Usually if I feel the need to swap handles - it's time to swap sails!
Got questions - just ask - be glad to answer!