Learn Hardcore Korean: banghak iljeongeun daebubun naega jeonghaesseunikka gongyuhalge. - I planned most of the vacation schedule, so I’ll share it.

방학 일정은 대부분 내가 정했으니까 공유할게. = “As for the vacation schedule, since I decided most of it, I’ll share it.” The object is introduced as the topic and understood in the second clause.
You could also say: 방학 일정을 내가 대부분 정했으니까 공유할게. Here, 일정을 stays as the object, with no topicalization. Both are fine; the first sounds more “as for X…” and is very natural in conversation.

What nuance does -으니까 have here? How is it different from -아서/어서 and -으니?

-으니까 gives a reason/justification from the speaker’s viewpoint and pairs very naturally with announcements, promises, or suggestions. It can sound slightly more assertive or persuasive.
-아서/어서 is a neutral cause/sequence link (“and so/so then”). It’s more descriptive and less about justifying a decision. With a promise/command that follows, -으니까 usually feels more natural.
-으니 is similar to -으니까 but feels a bit more written/formal or literary.

All of these can work here, but 정했으니까 공유할게 is the most idiomatic when you’re announcing a decision to the listener.

Why is it past tense 정했으니까 instead of 정하니까?

The decision is already made, and that completed action is the reason for the next action. 정했으니까 = “since I have decided.”
정하니까 would suggest a general/habitual or simultaneous situation (“as/when I decide”), which doesn’t fit well here.

What exactly does 공유할게 mean compared with 공유할 거야, 공유할게요, and 공유하겠습니다?

공유할게: Casual “I’ll go ahead and share (for you).” It’s a listener-oriented promise/decision.
공유할 거야: Casual future/prediction of one’s plan; less listener-oriented than -ㄹ게.
공유할게요: Polite version of the listener-oriented promise.
공유하겠습니다: Formal, strong commitment; used in announcements, meetings, or to customers/seniors.

Is the sentence polite? How do I make it appropriate for coworkers or seniors?

Without 요, it’s casual. For polite speech, add 요 and use 제가:

방학 일정은 대부분 제가 정했으니까 공유할게요. In a workplace (and if “vacation” is from work), say 휴가 and optionally add a benefactive:
휴가 일정은 대부분 제가 정했으니까 공유해 드릴게요.

Why 내가 and not 나는? Are both possible?

내가 uses the subject marker 가 and puts focus on the doer of the action—“I (as the one who did it) decided most of it.” It can subtly contrast with others.
나는 makes “I” the topic—“as for me”—and can feel heavier or off-target when you want to emphasize who performed the verb. Grammatically possible, but 내가 is more natural here.

What’s up with 내가/제가? Why not 나가/저가?

With the subject marker 가, first-person pronouns irregularly appear as 내가 (from 나) and 제가 (from 저). Forms like “나가/저가” aren’t used for the subject marker. Note that “나가” is read as the verb “to go out,” unrelated to the pronoun.

What is 대부분 modifying? Do I need to say 대부분을?

Here 대부분 means “most (of it)” and is understood from context (the schedule). Korean commonly omits the object marker in this pattern:

대부분 내가 정했으니까 ≈ “since I decided most of it” You generally don’t use 대부분을 here. If you want to be explicit, rephrase:
대부분의 일정을 내가 정했으니까
일정 대부분(을) 내가 정했으니까 You can also shift placement:
내가 대부분 정했으니까 (focus on “most”)
대부분 내가 정했으니까 (slight emphasis that it was “I”)

Is 공유하다 with 일정 natural? Any alternatives?

Yes. 일정을 공유하다 is very common, especially in digital contexts (sending a calendar link, posting details). Alternatives depending on context:

보내 줄게요/보내 드릴게요 (I’ll send it to you)
공유해 줄게(요)/공유해 드릴게요 (adds a “for you” nuance)
캘린더로 초대할게요 (I’ll invite you via the calendar)

Why is there no explicit “it” with 공유할게?

Korean drops repeated/obvious elements. Since 방학 일정 is already introduced as the topic, it’s understood as the object of 공유할게. If you didn’t set it as a topic, you’d say 일정을 공유할게.

What’s the difference between 방학 and 휴가 in this context?

방학: A school break (students/teachers). “방학 일정” = schedule for the school vacation.
휴가: Time off from work. “휴가 일정” = time-off schedule. Use this for workplace contexts.

Any spelling gotchas? Is 할께 okay?

Write 할게, not “할께.” The future/promise ending is -ㄹ게.
Write 정했으니까, not “정했으니깐.”
No space in 공유할게(요) (not “공유 할게(요)”).

Could I say 공유해 줄게? What nuance does 줄게 add?

Yes. -아/어 주다 adds a benefactive “do it for you,” sounding more considerate:

Casual: 공유해 줄게.
Polite: 공유해 드릴게요. Note: In business Korean, some say “공유드릴게요,” but the more standard form is 공유해 드릴게요.

Can I put the reason after the promise, like “공유할게, 방학 일정은 대부분 내가 정했으니까”?

You can in speech for an afterthought, but the neutral, most natural flow is reason first, result second:

방학 일정은 대부분 내가 정했으니까 공유할게.