Patur Aval Assur,
To that I ask that if you are serving Hashem ??? ?? ??? ???? ??? in order to get the “reward” of the relationship than that is no less self-centered/selfish than doing it ?? ??? ???? ???, and if you are doing it purely for Hashem’s sake, then why are you doing it? Why do you care what Hashem wants?
To do something for the sake of a relationship is not inherently self-centered.
I’ll was the dishes so that my wife will make me some yummy barbecue: ?? ??? ???? ???
I’ll wash the dishes so that my wife can have a stress free evening: ??? ?? ??? ???? ???
The challenge in using this marital model to describe our relationship with Hashem is, as you seem to be hinting at, that in reality, what we do has no “impact” on Hashem. He is eternal and possesses everything – we cannot benefit Him or harm Him in any way. So how can we do anything “for His sake”?
Despite that reality, Hashem desires to have a relationship with us, so He has provided a means for us to do things “for His sake” (the mitzvos). We, therefore, now have the ability to conduct that relationship in a more or less selfish manner. And that does have a real impact, because Hashem desires a relationship with us.
What if there was no relational reward either? Would you still serve Hashem?
The only way to not have a relational reward would be to not have the Torah – at which point we wouldn’t even know how to serve Hashem anyway and would render the question moot. This would essentially be a Deistic model.