I’ve tried several distros to fit on my repurposed Chromebooks that came with 16 GB emmc storage. Debian was the smallest one, using by default about 5.5 GB of data, plus 1 GB for swap, plus the boot partition. I had about 9 GB left after installing, with XFCE. After I installed a few apps and games, I ended up with 6 GB free space. It works fine and it updates fine.
Yeah, I don’t understand flatness either. Neither I understand the dark themes either. My eyes and brain simply can’t do the separation easily, I spend more time trying to process an image. Old style icons and UI colors are the best IMHO.