How? Policies regarding Russia haven't changed. It started in the Schroeder-era.

They could have kept the lid down on the situation and prevented the East European nations from inflaming the situation further. They could have offered to veto Ukraine. The new government did not defend German interests and let it become a pawn in great power politics.

I don't see how that would have either lowered Putin's desire to annex Ukraine or increased the cost of doing so.

Yeah because the totally competent CDU/CSU politicians would have solved this way better. Totally!

What can they realistically do? Send German troops there? It won't happen ever, we both know it. Shutdown Nordstream 2 earlier? It's unlikely to have any effect. Direct supply of weapons into Ukraine? Earlier and preemptive sanctions? None of that will deter Putin, in fact it will probably just encourage him to act faster.

You also have to take into consideration the last 2 times Germany entered a conflict it didn't work out too well for them, so they have all the more reasons to be reserved.