What are the key factors to consider when scaling a startup?

Team culture. That is the hard to maintain it while scaling.

Baby steps. Slow growth is better than a sudden one, because the latter can end just as suddenly.

Sustainable growth

Product/Market Fit: Before scaling, ensure that there's a real demand for your product or service. This is often demonstrated through consistent or increasing sales and positive customer feedback.

If you have found product market fit which is when I believe a startup turns to a scaleup, then you would need to hire as fast as you can. Its time to hit the gas. It might be a new market your product is trying to create, so you should aim to dominate the new market you've just created. And please, try as much as you can to instill a great culture in your Company.