Does anyone else here find vanilla css unscalable, to a point where it's just unusable

Definitely. SCSS all the way

Not sure what your use case is but it's possible that you're not using CSS to its full potential

I personally use CSS but split the styling into multiple files during development (as many as I need no matter how many). Using techniques based on modern css rules such as grid and flexbox makes it easy to scale css and write less styling code.

Finally, I find an automated way to bundle them before going to production.

SCSS is quite good as well if you find that easier.

If u lack best practice of course.

I kind of agree, but it definitely depends on how well you structure your project from the start. If you use classes, etc... In a good way from the get-go it works pretty well. But if you're unorganized it will bite you in the ass later, cause it's a real pain to refactor CSS.

It could get a little messy on hudge projects. The best is probably to do component based CSS, use SCSS and BEM methodology