and weaponry and won't lose the primary phase of the war.

The best case scenario is the sanctions (which are severe, and could be even stronger if we sanction gas/oil exports) steadily weakening and isolating Russia while they are mired in a guerilla war in Ukraine for years.

Sanctions will work because the Russians, especially younger, have become used to a more Western and consumerist way of life and won't be happy otherwise. It's not just about raw resources (Russia has enough food an oil to sustain itself) but about not wanting to live in a drab Soviet style existence.

Ukrainians will never accept the Russian occupiers, such a project is certain to fail.

Thus under such pressures, at some point the Putin regime will fall apart and someone else will come to power, who will relinquish Ukraine/Belarus/Georgia (and accept they will join NATO) in exchange for the West lifting the sanctions and accepting Russia back into the international community.