Asklemmy

I would expect multiple observers to have the same result no matter the distance between them.

It's not a matter of distance but of information isolation by additional layers of measurement. You can read on the unintuitive experimental result, a separate mathematical paradox similar to Bell's paradox with consistency as one of the three assumptions where one must be false, and a paper discussing the difference between foundational relative facts and their occasional emergence as stable facts.

A relative paradise for me involves my loved ones. If they would not be there as they are now in my life, then it's no paradise. But that would contradict our initial condition of ideal afterlife.

It's difficult to describe this topic without falling back on physical parallels, and frankly given the origins of physics and philosophy as having been hand in hand for millennia up until fairly recently, I disagree that it can't offer clarity.

In this case, a classical interpretation does seem contradictory because identity is unique. There's only one of each thing. But when we talk about entangled particles, they are mathematically identical. If we're discussing the notion of a simulated copy of an original reality fracturing into multiple ideal paradises relative to each individual, you can have identical versions of every person in your life in your relative copy of it while the ones you were around are each in their own splinter off worlds. So what's lost is a classical certainty of them being the same. But because it would be impossible to test or evaluate if they are the same or different on the other side, you functionally wouldn't know either way. With a quantum (or even just simulated) cake, you can have it and eat it too.

Accepting unconditional relative afterlife idea either nulifies any moral argument

Actually this is more broad, which is that accepting a fundamental relativity of all things nullifies any absolutist morality. To which I completely agree.

If I get to pick and choose things I accept in a theory, then it's a bad theory.

And the beauty of it not being confirmable in the here and now and relatively observed in a hereafter is that even if I'm right you'd be able to have your own experience of existence now or later completely different from what I'm proposing unable to discover that beneath the surface it is technically what I'm laying out above. Christians can think they are in heaven and everyone else is in hell without anyone actually being in hell or their idea of heaven being heaven for everyone, and you can have whatever existence or nonexistence you most desire without it crimping my or anyone else's style.

What moral foundation is there to back up the descision?

This is the same argument Christians make when they are confused that atheists don't commit crime when they don't believe in God having told them not to.

Morals don't inform behavior. We've invented morals to fit our preexisting socially adaptive behaviors. We don't eat babies because of our evolved biological desires, environmental necessity, and social consequences. Not because a potential baby eater is thinking over Kant's moral imperative. And in the rare instances where biology, environment, or society encouraged baby eating Kant didn't save those babies.