I'd say there's a good chance that you're equating correlation with causation, to an extent; it might not be your systematic self-improvement plan that's effective insomuch as you, yourself, have grown and matured over the past few years and are now more confident and self-assured with different needs and traits than those you had years ago. That, to me, seems like pretty typical individual maturation.
I'd question the inherent assumption that polyamory is inherently more ethical. While monogamy does limit some relationships from forming, it can also prompt deeper emotional bonds and trust between the partners within that relationship.
So too, it's unclear how long you've been in your current relationship. I'd question any analysis of "good relationships" that didn't look at quality over the course of a relationship for a significant amount of time. What "significant" is is debatable, but relationships tend to get harder as they go along, not easier, and if overall quality of one's relationships is one's goal, then initial quality would be, at best, misleading.
I'd certainly agree with your first point; I was more thinking in terms of the initial "high" most people feel within the initial period of a relationship. As time progresses and infatuation ceases, relationships typically become a great deal more "work" than they do when you're enamored and full of almost unconditional love. After getting used to that shift, though, I can certainly see variation over time. But I do firmly believe that the initial few months, for most relationships, are the easiest. Hence the need for an evaluation after the initial period of infatuation has faded [which it may have, but it's unclear in the OP].
I don't think polyamory is inferior to monogamy, but I think deciding between them ought to be a case of an individual recognizing their own needs and limitations. For instance, I'm not going to tell an abuse or rape survivor with significant trust issues that they just need to rationally overcome their learned fears; it may be a significant accomplishment for them to even thrive in a monogamous relationship with trust and no jealousy. Calling that "less ethical" seems to ignore individual context and adhere to an optimal reality that ours and the people in it simply can't match.