A lot of times, timid birds become more tame when able to fly because they feel more in control.
This has definitely been my experience! My budgies currently hate me because I've had to grab and medicate them.
who knows if I'll ever gain their trust now.
But I have a flighted cockatiel, Astrid, whom I adopted in April. She came to me clipped but very fearful of hands. From the beginning, I let her do her own thing. Come in and out of the cage at will. I attempted a little bit of step-up training with a perch but it made her too nervous so I backed off. I'd just hang out, offer her millet, and let her be. Her wings grew in and she learned to fly beautifully. Then one day, she flew over to me! Soon she was perching on my shoulder. She's still wary of my hands, but she's making steady progress. At first when she started flying to me, she'd fly away (as if to reassure herself that she could) and come back a minute later.
Being flighted and knowing she's in control gave her the confidence she needed to approach me. I have no doubt that clipping her wings would have done nothing but make her trust me less (also because grabbing and restraining her would have been involved). Keeping her in the cage until she became tame would have been cruel, as my other tiels have so much freedom...plus her being happy and getting lots of exercise made her much more agreeable. If she'd been stuck in the cage while I waited for her to learn step-up, she'd probably still be cage-bound after four months. She needed the opportunity to come to me
on her terms, and that's the foundation on which our trust has been built.
Also having tame birds around as an example for her really helped. Leon is tame, right, and Butch semi-tame? I'm sure your new keets will want to copy them.