You are on the right track Tom
You HAVE to start turning the kite simultaneosly with starting your carve/turn, to avoid the slack lines and to get a smooth carve.
And you have to lean into the carve, but not for making the turn, you have to learn to turn, so you can lean into the now not too wide turn.
Rubbish you might say ?
But I am serious - remember clearly when learning to carve, and leaned over like used to on surfboards or twintips, but the foil did not turn with me and I got thrown off, often ventilating too
Remember to think "Yaw", using your front foot to twist the board into the turn
When you master this, the "leaning into" comes automatically for keeping good balance and it feels like a surf carve now
If you do the opposite, lean into the turn to get it to turn, it will go really wrong - that is my point here
You can not carve having the mast vertical, that is for sure... But you should not think about the mast, think ONLY about your yaw impulse, which turns the hydrofoil in a curve, and mast (and you) will position itself correctly by itself, great.
The reason why it works having the kite around 45 or less and flying it up and around, is that when it flies up simultaneously with you going downwind, you avoid the slack lines and can keep tension all the way around a lot easier.
But I am sure your problem is still, that you dont carve "tight" yet, thus you get slack lines too easy...
You can try to downloop, but you might not gain much except being able to go in a wider turn, but probably a lot more often getting the kite in the water
I would wait with the downloop, till you can master a normal carve okay.