Reincarnation in Bleach: Buddhism lens.
The Soul Society is not Heaven in a western sense. It is a place where souls await reincarnation - somethings similar to Bardo. It is not the final destination for a soul, nor the reward for good deeds.
The karmic system is never stated in Bleach but is is implied for example: by existence of Hell (very bad karma), or beast clans like Komamura's (also bad karma). This karmic system controls and defines reincarnation. This is no luck that you are born poor or rich, weak or powerful - this is karma - determined by your deeds in your past lives. So when you're reborn a noble, rich clan member - perhaps you had a multiple cycles doing good deeds - and also, while you are reborn lucky, powerful, and rich and then you're not doing anything good with it - you are burning out your karma to actually get to Hell next (or any lower state of beings). Like, why powerful beings all go to Hell? Because they wasted their karmic privilege already.
And suffering that exists even after death - in Soul Society for example, has its purpose in karmic system. It is supposed to trigger you to look for getting free of it - breaking the cycle of samsara and reaching nirvana - end of suffering. Without suffering there is no purpose, motivation, to look for nirvana. That is why there has to be suffering in the 'afterlife'. Because Soul Society isn't the end goal of the cycle. Being outside the cycle is the goal.
In other words, suffering is a tool - it brings you closer to enlightenment. You won't better yourself if you never suffered.
That is why the Yhwache's vision of the world without death is faulty.
I believe there should be something beyond the Three Realms in Bleach cosmology that would equal the nirvana state, as we already encounter characters who broke themselves out from the cycle of reincarnation: Ichibei and Squad 0, and Aizen (all being immortal).
In that reading, they represent a version of Bodhisattwa state of being - beings who does not have to reincarnate anymore but still stays inside the cycle to lead other beings to their enlightenment.