For purely positive masculinity, Avatar is good. It was great for it’s time, but I think there are some bits and pieces that haven’t aged the best. Most of the female characters end up being damousels in distress. The most prominent, Katara, is primarily a maternal figure who mostly adheres to traditional heteronormative gender roles. Toph was originally written as a joke, according to interviews with the writers. It’s hard to say for sure how much was on the writers vs the execs at Nickelodeon. For the time, it was still fairly progressive.
Korra is a worse show for a variety of reasons, but this is one of the places where I think it’s better than Avatar. Another show that shares some writers with Avatar is The Dragon Prince. It’s still not done yet and it’s not perfect, but so far seems to be pretty good.
I do think Avatar helped to bush the boundaries and opened up doors for later shows. Adventure Time is probably the biggest, and I’d say Finn is a fantastic example of a boy growing up and learning to avoid toxicity. That spawned a wave of what some derrogatorily and erroneously refer to as the “Cal Arts” era, which I think are even more great examples of not just positive masculinity, but positive humanity. Steven Universe is probably the most relevant here, but it seems like almost everyone who worked on Adventure Time went on to get a show of their own after, and most of them are pretty good.