Russia and Ukraine

[Thumpalumpacus]

Again, the Baltic states joined in 2004, putting NATO right up on Russia's border, but this is now a bridge too far?

There are several key factors - at the very least would be geography and logistics. That said, I doubt it helped.

tl;dr - Baltics-to-Moscow = like fighting through Louisiana; Kharkiv-to-Moscow like fighting through eastern Colorado.

The closest major population center of the Baltics is roughly 535 miles away from Moscow and 1000 miles away from the industrial heartland of Russia; inbetween is one of Europe's largest chain of wetlands, marshes and swamps with direct access to only 1 major highway to the capital - outside of that it's almost entirely mud roads not fit for heavy machinery. Additionally there is a low amount of commercial traffic in the region and few population centers, making it ideal for supremacy of sea and sky  - not for staging a full expeditionary force on Moscow.

Adding Ukraine to NATO only reduces the distance to Moscow by about 100 miles; major industrial targets are now within 670 miles, with Tula - a major arms production center - within 300 miles of Kharkiv. In comparison to the border with the Baltics being a natural deterrent mixed with Belarus, the border between Russia and Ukraine is a well developed region - and one suited for military travel, as thousands of years of roving warbands in the region have bloodily proven.  

This not only opens the way for arial supremacy for NATO within Russia's own airspace, but also adds 3 more direct highways to the Russian capital and several more to the industrial regions; this surrounds Moscow on 2 fronts and allows the possibility of an easy encirclement to the east - stripped of any access to the oceans, deprived of it's productive and oil reserves, and backed against a frozen tundra.

This is not a "Cuba is close to America" issue - this is a, "Russian bases in Ontario" level of provocation from their perspective, agree with it or not.

I'm sure Putin was aware of things like attack jets and SRBMs that could be based in the Baltics in 2004. It's not like we're stuck with 1942 technology. But since you want to go down a rabbit hole and look at geography, go take a look at a map, and then imagine what controlling the Baltics might do to Russian maritime trade coming from St Pete.

Viewing NATO as an existential threat to Russia is silly. If NATO wanted to snuff out Russia, it could have done so with ease in 1996, yet it didn't.