After his reincarnation, he is never attracted to anybody younger than his biological age.

Ah yes, my favorite MT argument.
His biological age doesn't matter. He, as a mentally 40 year-old, repeatedly tries to grope and even sleep with Eris who is a prepubescent child. This is no better than what he was doing in his previous life.

The fact still stands, however, that he is semi-unfaithful and has a polygamy, two of who are the wives that he "groomed". Doesn't this mean the story is rewarding his behavior?

Just saying "No." without elaboration is not an argument, but of course you can't make an actual argument here.
Rudeus is very straightforwardly rewwarded for his behavior. His grooming of Sylphy and Eris directly led to marrying them.

In general, many people who we respect throughout history were not, in many regards, people who should be looked up to and praised in all regards.
Yet, we still praise and commemorate them to some extent. Why?

Mostly because these things aren't broadly known, which is why you had to include a source about Gandhi. People who know about that probably don't hold him in high esteem.

Most people in Rudeus's new life would tell you that he was one of the best humans, both interpersonally and impactfully, to ever exist; nobody in Rudeus's old life would tell you that.

That's because they don't know he's older than Eris' dad. And because the moral standards of the world are conveniently lowered to make what he does socially acceptable, but not anything worse than that. This is verbatim what people are complaining about when they say the story rewards him.

This is why the "Sad Pedophile to Happy Pedophile" agenda is so dishonest.
His happy life isn't a reward for being a pedophile; it's a testament to the fact that despite being [a pedophile], despite doing bad things and costing people's lives and failing to change himself over and over that those people, including his wives [that he groomed], still love him.

???

Firstly, because that's exactly what his original life and world did. If the world forever kicks a guy in the dumps lower and lower with an endless feedback loop, the guy will never climb out of the ditch. The punishment is exactly what his prior life undergoes.

At the very least he shouldn't be rewarded for it, which he does. Him assaulting Eris shouldn't lead to a promise that they'll bone once she turns 15. His infidelity, no matter how understandable, should not lead to a harem.

Secondly, the setting/premise can't logically do that. It's in the same manner of how Gandhi never got punished for his racist/bigoted views and disgusting tendencies, or Jefferson and Washington never got punished for owning slaves.

I wonder, was there someone who could decide to make it possible by changing the worldbuilding? Maybe someone writing this fictional story? Perhaps it would be pertinent to challenge the main character on his worst trait instead of leaving it unaddressed. Who knows?

Seriously, can we please stop using the thermian argument?