It's not gonna happen. Putin will hold those territories as long as he lives. Ukraine will agree to a ceasefire on current lines but will never officially cede territory to Russia. If Europe keeps sanctions on until a full withdrawal occurs it can lead to an eventual withdrawal when Putin dies.
So according to this logic, the priority must be on his rapid demise
The best bargaining tool the West has is the 100s of billions in frozen assets. Something the next guy along will desperately need to fix their broken country along with sanctions relief and business deals with the West. Using that to get a full withdrawal from occupied territory is possible.
There's no scenario where Ukraine retakes occupied territory by force (too many mines/fortifications) nor is Putin ever going to leave occupied territory as long as he lives. Will their economy collapse forcing a withdrawal? Possible but unlikely even though their situation is dire.
There is such a scenario - several in fact. If western troops on the ground are ruled out, then the west just supplying Ukraine with all necessary weapons and intel would suffice. The scenario is real, even if difficult given russia’s hold over / kompromat on orange Sphincter Mouth