I second nemo. Gnome makes it annoying to swap file managers since you need nautilus installed for the desktop icons so you can't just remove it. Nemo has its own but they aren't as good on gnome.
To swap away from nautilus after installing (any file manager, but in this example) nemo you use xdg-mime:
Welcome to Gnome. "The defaults are good so you don't need customization" seems to he their montra. Honestly there probably is a way to change the sidebar however you like, but it will he buried in a config file and break between upgrades. Dolphin might be a better fit. KDE seems to go the other way, not great defaults, but you can customize as you want.