In the early morning, I came across several pictures of people working overtime late at night on my former colleague's WeChat moments.

PThe people in the pictures were very exhausted.

New Year's Eve wais supposed to be for celebration in modern traditions, but they had to work overtime late at night just because of their high salaries.

I think working overtime too much is disruptive to our health, so much that it may result in us ending up working to pay for hospital bills.

Or "and" but "so much that," or "to the point that" emphasize the cause and effect relationship

In the early morning, I came across several pictures of my former colleagues working overtime late atinto the night on my former colleague's WeChat moments.

OR: In the early morning, I came across several pictures of people working overtime late into the night on my former colleague's WeChat moments.

PThe people in the pictures were very exhausted.

You could also say: "The people in the pictures looked very exhausted."

New Year's Eve was supposed to be for celebration in modern traditions, but they had to work overtime late atinto the night just because of their high salaries.

I think the sentence would sound better without this section. If you wanted to add it, I would phrase it as: "In modern times, New Year's Eve is supposed to be for celebration, but they had to work overtime late into the night just because of their high salaries."

I think working overtime too much is disruptive to our health, and it may result in us ending up working to pay for hospital bills.