If final Nibbana is the cessation of the Khandas…? A question I've been pondering

You’re talking about nibbāna at death, I assume, parinibbana. The traditional view of the Theravada is that it is indeed the cessation of the khandhas. The Paṭisambhidāmagga literally states: “The cessation of the five khandhas is permanent extinguishment (nibbāna).” (Ps 2.238) There’s more quotes like this, although I have to say I’m not the biggest expert on the commentaries. The Visudhimagga and Milindapanha also equate nibbana to cessation, not to a thing.