That's a myth. They initially had only one season approved when they first started working on the show but then got approved for all four books before the first season even aired.
It was pretty nutty the way they kept yanking it around the network but fundamentally, yeah, the production wasn't all t hat complicated. The first season is the only one th at really suffered from this, as they were given the greenlight for the next seasons after it was too late in production to make any changes. I give Season 1 a little grace for this (although that in no way forgives the decision to tell the story the way they did, if they
knew they only had the twelve episodes) but the others really have no excuse (except that they fucked themselves by resotirng her bending at the end of Season 1, I know I'm not th e first person to notice that it would've been an easy up too have the following seasons be about her actually learning the spiritual aspects of bending now that she's been stripped of her physical power).
I don't see it. Aside from that one episode at the start of book one where she challenges Amon to a 1 v. 1 and he predictably doesn't acquiesce to this stupidity and shows up with a whole army of Equalist goons, none of Korra's decisions ever backfire on her. They seem impulsive and short-sighted to the audience but story-wise they work out basically all the time.
Right, and this was my point. It's not that Korra 'lacks strength', it's that the writers didn't seem to understand what those strengths actually are, so Korrra is more or less never met with any real friction. Korra
is implsive, but character flaws are irrelevant if the story never meaningfully pushes back against them.
An argument could be made that her inability to airbend was supposed to be a pushback and she would need to overcome her brashness and ego in order to become a true Avatar, but the resolution was arbitrary and didn't actually force her to confront her weaknesses, which leaves the impression that from a narrative perspective, Korra doesn't really need to develop.
You mentioned Mary Sue and I agree. A lot of what people point to as indications of Mary Sue are common symptoms of an underlying story failure. Mary Sue isn't just super powerful and really talented and beloved by everyone and hot, the story bends over backwards to make sure she encounters minimal, meaningful challengul challenge, and what challenge she does encounter is usually turned to enforce sympathy than real growth. Korra's mercury poisoning could've been a really cool thing for her to overcome: we missed out on her having to re-learn the spiritual aspect of bending, but we could still really dig into who Korra is when denied prowess as Avatar.
But that's hard and it means Korra has to try and we don't get cool fights so... eh.