have you considered keeping the root zone in a separate, lower section? the additional volume at even the same height has your pots might be worth it(?)... and another thing what about (alternatively) using shorter, wider pots like mini kiddie pools, or even a shallow bed?
i'm pretty sure you're aware of how closely Bonsai maintenance parallels to your Besta setup, and that's a great example of a small container, mature plant scenario.
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