Japanese Black Pine from seed

Hi Jonas. I have several Japanese Black Pines that I planted from seed. This batch will be 4 years old in spring (September). I’m in Cape Town, South Africa. After the first year I planted them in pond baskets, then the following year, I placed the pond baskets in to colanders. Free draining soil used throughout. I’m concerned as most of the needles are yellowish instead of dark green (old and new needles). I have been fertilising spring to fall, holding back in winter. Some do not drain well when watered though. Should I repot these by just removing a little off the top, sides and bottom or would I need to do a more severe repot to really improve percolation?
Secondly, would you advise to leave the long leaders or at this stage, or cut back to a new lower leader, and hopefully generate back budding on the lower part of the trunks, which will better thicken the base?
I really appreciate your blog posts, they’re very inspiring!
Best regards, Mark

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