Russia wants to conquer Ukraine and the EU wants to prevent it. While the ways to do it diplomatically may have been wrong and therefore failed, it primary is the responsibility of the aggressor (Russia) for high count of casualties both Russian and Ukrainian.
>> Unmistakably, this is a massive tragedy for Ukrainians and I think it was avoidable.
What exactly you think should have been done to avoid this?
NATO could have resisted expansion to the East.
A federalized option agreeable to the Ukrainian secessionist provinces promising some autonomy might have been successful.
The West could have declared an eastern European buffer zone between NATO and Russia. ("Little Entente" v2.0)
Some sort of internationally-monitored plebiscite in the Ukrainian secessionist states and Crimea.
There's thirty plus years of historical context that's boiled over into a shooting war, but that context is important for understanding where we are now without relying on simplistic abstractions like nationalism or "democracy".
2) That's incorrect. It wouldn't have stopped Putin but only given him more than Ukraine.
3) Probably had little impact on Russia's decision to start a war.
4) The same as (2). It wouldn't have stopped Putin but only encouraged him even more to subjugate Ukraine, if it didn't want to play by Russia's rules.
In other words, I don't think you have any suggestions that would have helped to avoid this war.