I think the real red line was the use of a color revolution to seize Ukraine by force.
This is a point I've raised a few times. In an alternate history we don't know how it would play out if Ukraine peacefully and democratically abandoned Russia for the EU. Would Russia be provoked into attempting to cling to their sphere through oppressive tactics, coercion, invasion? Maybe, maybe not. Russia lost a creeping influence to the west in soft NATO expansion, but that fumbled in Ukraine and instead the west backed an overt nazi led coup d'etat like it was some CIA operation in a banana republic. On Russia's doorstep that could not go unanswered.
This also aligned China to Russia's side more than any other misstep Biden could make. Nothing China fears more than our soft power and shadow ops trying to destablize them and their sphere and their extreme distaste for color revolutions. They had no dog in this fight and instead lent their silent support to the Russian side.
It's more complicated than just a sphere of influence. Crimea had always been Russian with an important (ice-free) Russian naval base in Sevastopol, a large group of ethnic Russians live in Eastern and South Ukraine, important cities like Odessa were founded and built by Russians. We're talking about a historic part of Russia that became Ukrainian when the USSR broke up and Russia simply wasn't gonna hand it over to NATO and have NATO warships in Odessa and Sevastopol right on their border.
About the timeline, this is a great article to read, it explains everything
https://carnegieendowment.org/russia-eurasia/posts/2016/04/the-revival-of-the-russian-military(Quotes from linked article)
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After the collapse of the Soviet Union, the Russian military rotted away. In one of the most dramatic campaigns of peacetime demilitarization in world history, from 1988 to 1994, Moscow’s armed forces shrank from five million to one million personnel. As the Kremlin’s defense expenditures plunged from around $246 billion in 1988 to $14 billion in 1994, according to the Stockholm International Peace Research Institute, the government withdrew some 700,000 servicemen from Afghanistan, Germany, Mongolia, and eastern Europe. So much had the prestige of the military profession evaporated during the 1990s that when the nuclear submarine Kursk sank in the Barents Sea in 2000, its captain was earning the equivalent of $200 per month.
NATO took advantage of this. They'd promised they wouldn't expand and kicked the Russians when they were down. The Russians could only protest -->
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Moscow protested vehemently against the alliance’s eastern expansion, its 1999 bombing campaign in Yugoslavia, and the 2003 U.S. invasion of Iraq, but Russia was too weak to block any of these moves.
In 2008 Putin announced a massive increase in defense spending which was the only option left when a powerful military alliance moves against you, breaking their earlier promises.
To rebuild a military that had completely crumbled would take a long time, I think that they weren't ready in 2014 but felt forced to act because of the Maidan coup. It explains why the Russian military performed so poorly at the beginning of the invasion
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Beginning in 2008, Putin ushered in military reforms and a massive increase in defense spending to upgrade Russia’s creaky military. Thanks to that project, Russia has recently evinced a newfound willingness to use force to get what it wants. First, in February 2014, Moscow sent soldiers in unmarked uniforms to wrest control of Crimea from Ukraine, implicitly threatening Kiev with a wider invasion.