Wow, you have made great progress with all of the cats. I am sorry to hear about Tuki but happy to hear about Klein. In my experience, feral cats have a tough time being picked up. To condition Klein for this, start by lifting one leg off the ground for a few seconds while rubbing his chin. When he is comfortable with that, pick up the front half of his body but leave his back feet on the ground. Eventually, you will be able to pick him up entirely. If the goal is to bring him inside, I would not introduce him to the larger enclosure until after he is comfortable in the house. To keep Zambezi and Calypso safe, I would introduce them to the large enclosure. I would not trust them outside on there own.
Overall, you have done a remarkable job with these cats. Thanks for the update and please keep up the good work.