When Russia occupied Crimea, Ukraine pulled out all support for the region's infrastructure. (Electricity, etc.) Quite naturally, Russia had to foot the bill to replace it.
The only way to get around that is to seize the entire country.
You cant just occupy country w/o sanctions, export/import chains breaking, you need to rebuild what would be inevitably destroyed during the invasion, etc.
Ukraine shows how much can be tolerated.
Well, Russian economy is still in recession. And Russian forces tecchnically never fired a gun in Ukraine or Crimea.
Russian forces have fired many guns in Ukraine, full stop.