If russia could conquer ukraine by wanting it harder, I think they’d have tried that? Russia has stalled and began losing ground because their army isn’t very good, not because of some detail of russia’s desires
They haven't tried that with all means available to them, and the means aren't nukes. As the recent events show, Russia could put Ukraine into a total energy blackout [1][2] if they decided to pursuit this strategy of waging the war. This would be the same strategy that was used by the US military in past conflicts [3], and it implies significantly more civilian casualties in a long run.
[1] https://www.bbc.com/news/world-europe-62873205
[2] https://www.theguardian.com/world/2022/sep/11/russian-strike...
[3] https://media.defense.gov/2017/Dec/29/2001861964/-1/-1/0/T_G...