How can I make the draggable titlebar file icon always appear at the top of files in Big Sur?
If you upgrade to macOS Monterey (at least beta 3), you can make the icon always show:
System Preferences → Accessibility → Display → Display (tab) → Show window title icons
Enable this from the command line by setting
defaults write -g com.apple.universalaccess showWindowTitlebarIcons -bool YES