た途端とたんに

とき just places two events together, with no claim on closeness (座すわったとき is neutral). とたんに needs the second event almost at once, and reads as a jolt (座すわったとたんに椅子いすが壊こわれた).

たとたんに needs the first thing fully done before the next starts (着ついたとたんに). か〜ないかのうちに blurs that line, so the first might not be finished yet.

とたんに just says the second thing follows the first, surprising or not (着ついたとたんに). かと思おもうと needs a sudden switch, usually the opposite (泣ないているかと思おもうと、もう笑わらっている).

たとたんに reports what happened right after, often a surprise (ドアが開あいたとたん、猫ねこが逃にげた). What follows 次第しだい must be a non-past planned act, never a surprise (分わかり次第しだいご連絡れんらくします).

とたんに needs the first event instant, with the second right behind (×長時間ちょうじかん考かんがえたとたんに fails). 末すえに needs the first to take a long time.

たとたん pins the second event to the instant the first finished. すぐ is a bare word needing nothing before it, so only たとたん carries surprise.

とたんに needs the second event uncontrolled and surprising. と同時どうじに just claims the two coincide, and takes a scheduled second event happily (卒業そつぎょうすると同時どうじに就職しゅうしょくした).