Although, I had watched the series through before that.

A frequently made mistake.

Through means going or moving from one side of something to the other.

Though (without the r) expresses a contradiction or despite the fact.

Although, I had watched the series through before that.

This is okay, but it would be more common to write one long sentence: 'I watched it today, although I had watched it before that'.

Once I started to play the anime, I couldn't help but continue watching.

You need to complete this phrase as 'I couldn't help continuing watching' or 'I couldn't help but continue watching'

I watched the third story arc of Jojo's Bizarre Adventure 'Stardust Crusaders' on Netflix today.

This is grammatical, but the wording is potentially ambiguous. To me, it could mean either:

- "I watched the third installment of Jojo's Bizarre Adventure, Stardust Crusaders." In other words, you started watching "Stardust Crusaders," which is the third part of the franchise. This is what the original sentence in Japanese implies.

- "I watched the third narrative arc of Jojo's Bizarre Adventure: Stardust Crusaders." In other words, you watched what you identify as the third narrative thread of "Stardust Crusaders," for example, the part of "Stardust Crusaders" that takes place in Egypt. This is what the sentence in English implies to me, because of the punctuation and wording.

Although, I had watched the series through before that though.

"Although" always introduces a subordinate clause, so the sentence sounded incomplete when it was included.

Once I started to play the anime, I couldn't help but continue to watching.