I would still base it on the Immobile Bane instead of creating a new bane and I would ditch the whole damage transfer part because that sounds broken. I like the idea of using a contested roll to get out but that aspect also makes the bane really potent, so instead, I would propose a two-step bane:
First step: Suck In
Essentially the Immobile bane but you are halfway stuck in the Slime. Getting out is a regular resist roll.
Second step: Swallow Whole PL6 Alteration
You envelop your target totally and completely. If they were sucked in before, you get 1 advantage to Swallow Whole them. If they weren’t sucked in before you get 3 disadvantage to apply this bane. Breaking out requires a contested resist roll (Might or Agility) against your Alteration. The Duration only ends once the creature has broken free and the creature starts taking damage while being swallowed whole once they have to spend more rounds inside than their Fortitude score. Damage equals your Alteration score.
This is what I would start with and refine it some more.