In the end, I couldn't go with the "greater good" of the North or the greater good of the South but had to play how I felt Geralt would do it. Geralt wouldn't betray Roche and he wouldn't assassinate Radovid knowing it would result in Nilfgaard winning. It was a choice of evils and Geralt said it best.
Well not getting involved is ultimately a choice perfectly in line with Geralt as a character, since yes he wouldn't betray Roche, there's still one problem with your logic.
My Geralt will deal with Radovid when the time comes but after the war is won. This also has the helpful ending of getting Emhyr assassinated, which means Geralt just needs to get rid of Radovid and Phillipa Eilhart to guarantee Witcher Ciri's safety.
Aren't you being selfish , thousands of lives being destroyed by Radovid himself ain't enough, you gotta ensure thousands more die by killing the last remaining crowned head who has no heir and would plunge the north into another crisis like Temeria was in TW2 resulting in thousands dead.
Perfectly in line with Geralt of Rivia who'd watch the world burn before he let anyone harm Ciri but your Geralt would fail. It takes Dijkstra's considerable pull with the underworld in Redania to ensure the city guards don't save Radovid and it takes Roche and all his men to fight Radovid's personal guard.
Even if Geralt managed it he would be highly unlikely to walk away, and then what? Someone else comes to power or Nilfgaard under Voorhis will come back in and conquer the north as it happens in canon after Emhyr is removed .
Ultimately even under this scenario, which I call wishful thinking at best, Ciri would still remain hunted by those seeking to use her whereas in the Empress ending she realizes that she has to stop running and make a real stand and she can only do so as the rightful ruler of the Empire.
One of the key points of the main story is that Geralt can't hold Ciri's hand and protect her, she needs to do that herself on her own. Trying to hold her hand like a little girl will only get her killed.