I used CSS tooling like LessCSS and SASS for years before switching back to regular CSS because I too wanted to simplify the build process (i.e. by not having one), and I found I didn't really miss the nesting so much. Mostly I solved it by writing things fairly concisely as:
#menu { display: flex; justify-content: space-between; orange; border-bottom: 1px solid red;
background-color: orange; color: #111; }
#menu >a { font-size: 1.2em; margin-right: 1em; border: 1px solid #000; }
A bit of an unconventional way to write it, but I found it really helps keeping a bit of overview because so much more fits on your screen, and because CSS is fairly simple the extra density doesn't hurt readability all that much IMO.
The biggest thing I missed was variables; but that's been solved now.
Now I just want line comments (// ..). I dislike /* .. */ comments with a passion.