途中で details

write + (and…) + partway through

kaite irutochuude

partway through; on the way — a stretch broken into before it reached its end

meaning: in the middle of writing

本ほんを読んよんでいるとちゅうで寝ねてしまった。

hon o yonde irutochuude nete shimatta.

I fell asleep in the middle of reading my book. neutral

映画えいがの途中とちゅうで寝ねちゃったんだよね。

eiga no tochuu de nechatta n da yo ne.

I ended up falling asleep partway through the movie. casual

Examples

partway through — something cuts in while the action is still running

本ほんを読よんでいるとちゅうで寝ねてしまった。

hon o yondeirutochuude nete shimatta.

I fell asleep in the middle of reading my book. neutral

映画えいがの途中とちゅうで寝ねちゃったんだよね。

eiga no tochuu de nechatta n da yo ne.

I ended up falling asleep partway through the movie. casual

会議かいぎの途中とちゅうで失礼しつれいいたしますが、少すこしよろしいでしょうか。

kaigi no tochuu de shitsurei itashimasu ga, sukoshi yoroshii deshou ka.

Forgive me for cutting in partway through the meeting, but might I have a moment? formal

on the way — a point on the route between here and where the subject is heading

帰かえる途中とちゅうでパン屋やに寄よってくね。

kaeru tochuu de pan'ya ni yotteku ne.

I'll stop by the bakery on the way home. casual

学校がっこうへ行いく途中とちゅうに、小ちいさな神社じんじゃがあります。

gakkou e iku tochuu ni, chiisana jinja ga arimasu.

There's a small shrine on the way to school. neutral

途中に — に in place of で puts something ON the route rather than breaking the trip — 駅えきへ行いく途中とちゅうにコンビニがある.
途中まで — names how far the action got before it stopped — 本ほんを途中とちゅうまで読よんだ says the reading was abandoned there.

最中に
最中さいちゅうに catches the activity at its peak and dramatizes the intrusion, so it wants an interruption worth reporting; 途中とちゅうで is the neutral everyday word for any point partway along, and it alone covers a physical route (帰かえる途中とちゅうで), which 最中さいちゅうに cannot.
あいだに
あいだに views the interval from outside and lets a DIFFERENT subject act inside it (留守るすのあいだに); 途中とちゅうで stays inside an action its own subject is already performing and marks where it broke off.
〜かける
〜かける marks the break on the VERB itself and says the action barely got going (言いいかけてやめた); 途中とちゅうで is an adverbial outside the verb, so it locates the break anywhere along the stretch and can hang off a noun (会議かいぎの途中とちゅうで) where かける has nothing to attach to.

ついでに
ついでに hangs a second errand on an action already being done for its own sake and takes any volitional verb or a noun (掃除そうじをしたついでに); 途中とちゅうで only locates a point along a trip already under way, claims nothing about convenience, and on a dictionary verb wants a verb of going.