How can I make the draggable titlebar file icon always appear at the top of files in Big Sur?
Backup your plist first. ~/Library/Containers/com.apple.Preview/Data/Library/Preferences/com.apple.Preview.plist
Then run this command:
defaults write com.apple.Preview NSWindowSupportsAutomaticInlineTitle -bool false

It will change the look of the toolbar so the title name is now on its own row above the tool icons. You can revert back to default with this command:
defaults write com.apple.Preview NSWindowSupportsAutomaticInlineTitle -bool true

Alternatively you can change the delay setting from the default 0.5 seconds to 0 seconds with:
defaults write com.apple.Preview NSToolbarTitleViewRolloverDelay 0