He made many blunders. Broadly, I think the biggest ones were adopting a 'salami slice' strategy to bleed Russia long term to try and leverage their hubris about continuing this bloody war long after it was obvious it wasn't going to go the way russia wanted it.
Incremental escalation of weapons systems to keep Russia expending it's irreplaceable army hardware, so that they are neutered in the long run. As opposed to a quick, overwhelming NATO response where Russia retreats, gets the propaganda 'win' that they were indeed fighting all of NATO, and that their decision for restraint was to preserve global peace, and their defeat to an inferior 'little brother' race was not proof of their actual corrupt weakness. This isn't a bad strategy at all - if it works. But that leads to the REAL blunder - never planning a successor when it was clear he would be a one-termer, and assuming that Putin's cock-holster Trump could never win again. The classic Boomer problem, staying too long and failing to make plans for the next generation.
No-Fly zones are logistically significant, but not undoable. Totally agree with you, a no-fly zone is the end of the war. And Putin was not going to use nukes or attack anyone else, we can clearly see that now. Biden fell for escalation bluffs, but I also think his strategy was always going to be 'let's make them bleed themselves out', but again, under the bad assumption that his party wouldn't lose the next election to a brazen Russian asset.
Have to constantly remind myself - this is not where Russia wanted to be. And that's a good thing, even with all the suffering in mind.
We'd all like to have a movie-style exploding Death Star style ending where a fleet of A-10's and F-35's comes screaming across the horizon to the soundtrack of Rock You Like A Hurricane, and the Russians scurry back to mordor, but that's not realistic.
Russia has effectively been stopped. They are the ones talking peace terms. Their negotiating position went from TOTAL CAPITULATION to 4 ANNEXED REGIONS, DISARMAMENT & NEUTRALITY to 4 ANNEXED REGIONS PLUS CRIMEA to "Let us keep what we have", in about 6 weeks' time. They aren't mobilzing on a wide scale anymore, they are only continuing with terror bombing, and they're taking counterpunches whose effects are impossible to hide even from their flaccid sheep population.
This is the endgame. Which is rarely viscerally satisfying to either side. The fact that Zelensky can reply with a flat "No" to absurd Russian & Trumpian demands to cede territory means that Ukraine is in a better position than Russia. Now, and going forward.