Bogertophis mentioned the Antaresia pythons (Stimson's, Spotted, Children's, and Pygmy). They aren't Colubrids but they tick a lot of the boxes you mentioned. A Stimson's or Spotted will be 3' (Children's and Pygmy are smaller). They're very popular in their native Australia, but not a super popular snake in the U.S. for reasons I will never understand, because they are such a pleasure to keep.
They CAN have a super-intense feeding response, though, so if you get one like that you'd need to be willing to use hook training or target training if you want a snake you can just hand to friends. I have four Antaresia, and I'd hand 3 of them to anyone, including a child. The fourth thinks anything that's warm should be food, but I'm working with him!
Between the fact that you want a docile species and prefer to use tub enclosures, I kinda suspect you might be happy with a Plains Hognose. It's hard to convey how charming they can be; they are really sweet and funny/quirky snakes.
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