How can I say that is using colloquial expression?language? (or How can I say this in casual speech?)

"colloquial" is a fancy word for informal or casual. It's ok but given the tone of the writing, it feels a little stiff.

It’ll be uploaded atavailable this Thursday on YouTube, Netflix, or something.another service.

You could also say it will be drop this Thursday on YouTube and Netflix. If it will show up on both. If you are looking for casual speech, "drop" would be a good word to use (for American English).

I’m so sad that it iswill be the final episode.

What you have is fine, but you can also say it this way. If it's the final episode of the season, but not the series, then you could say "I'm so sad that this is the season finale. "

AI also I wonder what will happen in the next storyarc.

I'm not familiar with this anime series. When we say final episode that usually means the end of the whole thing. If it's the last episode of a season, then it's the "season finale." Which if this is the case, it would be better to say "next arc."

I worried it can finish properly cause it has about three minutes but it has a lot of foreshadowings.

I'm not sure what part of the ideas there are more important so rather than change the sentence. I would say it like this "I am wondering how the show will wrap up. There are a lot of loose ends."