Navalny, defacto Russian opposition leader (jailed), has an organization that has been doing internet polls on the support for war. See [0] for recent results. As it's an internet poll, and skews young, urban and liberal, it should provide a lower bound for the support of the war.
How does an internet poll conducted by an organization working for an already jailed opposition leader overcome the polling-in-authoritarian states problem of people not trusting the polling not to be monitored and not to be a black mark in a government file if someone doesn't like the answer?
You do have a point. But those people would rather avoid answering the poll in the first place. Those answering the poll are self-selected for strong anti-war sentiment.
The methodology is a bit a of a mystery, perhaps I'm wrong to assume this poll was conducted on an unaffiliated internet property, so no connection to the opposition leader would be evident.
In any case these are the most anti-war numbers of any poll out there, and still over 50 % put the blame on Ukraine and the West.
[0] https://navalny.com/p/6615/