Learn Hardcore German: Ich bin dafür, dass wir heute früher beginnen.

dafür sein is a set phrase meaning to be in favor or in support of something.
Here, dafür points forward to the content of the dass-clause (the proposal that we start earlier today).
Grammatically, it stands in for something like for that idea/proposal.

Why is there a comma and the conjunction dass?

dass introduces a subordinate content clause (what you are in favor of).
In German, a comma before dass is mandatory.
The dass-clause has the finite verb at the end, which is why you see the word order you do.

Why is beginnen at the very end of the clause?

In subordinate clauses introduced by dass, the finite verb goes to the end.
With wir, the finite form is wir beginnen, which happens to look identical to the infinitive, but it is a finite verb here, placed clause-finally because of dass.

Can I use a zu-infinitive instead of the dass-clause?

Yes: Ich bin dafür, heute früher zu beginnen.
Use this especially when the subject of the action is the same or obvious from context.
If the subject is different or you want to state it explicitly, prefer the dass-clause: Ich bin dafür, dass sie heute früher beginnen.

Why is it früher and not früh?

früh = early (absolute); früher = earlier (comparative, relative to some usual or planned time).
Since you mean earlier than usual/than planned, früher is the correct form.

Is heute früher the normal phrasing? Could I say früher heute?

heute früher is the standard way to say earlier today or earlier than usual today.
früher heute is generally not idiomatic; avoid it.
You can also say heute früher als sonst to make the comparison explicit.

What’s the difference between beginnen and anfangen here?

Both are fine; beginnen is a bit more formal, anfangen more colloquial.
Main clause: Wir fangen heute früher an. (separable)
Subordinate clause: … dass wir heute früher anfangen. (the verb is unsplit and at the end)

Can I just say Ich bin dafür. and stop there?

Yes. If the proposal is already known from context, Ich bin dafür. means you support it.
The full sentence with dass explicitly states what you support.

Can I omit dass like in English?

No in standard German. You need dass (or use a zu-infinitive construction).
Colloquial speech may drop dass in some dialects, but that is not standard and should be avoided in writing.

Do I need the comma after dafür?

Yes, before a dass-clause the comma is obligatory: …, dass …
If you use a zu-infinitive group (… dafür, heute früher zu beginnen), a comma is standard and recommended.

What is dafür grammatically?

A pronominal adverb (formed from da
- preposition für).
It stands in for a prepositional phrase referring to a clause or idea. Similar forms include darüber, damit, daran, dagegen.

What’s the opposite of dafür in this pattern?

Can I rephrase it with a noun instead of a clause?

Is eher a valid alternative to früher here?

Not for scheduled start times. eher often means rather/sooner in a different sense.
Use früher to mean earlier in the day or earlier than usual.

Can I front something else to position 1 (V2 rule), e.g., start with Heute?

Yes: Heute bin ich dafür, dass wir früher beginnen.
German main clauses are verb-second; moving Heute to the front moves bin to position 2.

How does Ich bin dafür, dass … differ from Ich bin damit einverstanden, dass …?

dafür sein expresses support or preference; it’s neutral and common in speech.
mit etwas einverstanden sein conveys explicit agreement/consent; it can sound a bit more formal or official.

Should dass be spelled with ß? Why not daß?

Modern standard German always spells it dass with double s. daß is the pre‑reform spelling and is no longer used in contemporary standard writing.