Let me spell out the difference as succinctly as I can: Assimilationism asks someone who has not integrated their identity into the uniformity of a majority. Pluralism tells that person to assert and cherish their unique differences. Integralism invites someone who has already integrated their identity (as whatever) to evolve into a more comprehensive sense of who they are that is less limiting.
At a certain stage in development, it’s not only acceptable but necessary for minorities to reject a uniform identity imposed upon them from without, and assert a strong identity of difference. At a further stage of development, that person may evolve into a more expansive sense of self without disowning any part of themselves. Ultimately, even this more expansive sense may fall away into a sense of identification with all sentient beings throughout Gaia, all beings existing in all dimensions of all worlds, and even a Supreme Identity, Godhead, Kosmos, or Ultimate Reality. That’s what the mystical thread in the world’s religions tell us, and that is my spiritual belief.
To understand the difference, it is critically important to first recognize that the self evolves in stages of increasing complexity and depth; otherwise these distinctions will be perpetually misunderstood and attacked from a place of ignorance.