Why did it take Russia 7 months to figure out to do this?
Putin remembered how easy it was to take Crimea in 2014. So he was probably overconfident. The Ukranians have adapted since 2014 and Putin wasn't prepared for that.
“Why did it take Russia 7 months to figure out to do this?”
It was not so much a delay in figuring it out (Putin attacked civilian infrastructure in Grozny and Aleppo), as it was a delay in resorting to it.
Reconstruction costs are huge after such a strategy, as is enduring hostility among the population. It is also a blatant war crime under the Geneva and Hague Conventions, that Russia is a party to.