Prevent Finder to launch on boot
Just disable the LaunchAgent com.apple.finder at /System/Library/LaunchAgents/

with sudo launchctl unload -w /System/Library/LaunchAgents/com.apple.Finder.plist and Finder shouldn't launch at all.

Depending on your system this might not be successful. Then download, install and open LaunchControl. Navigate to System Agents -> com.apple.Finder and highlight it. In the menubar choose Job -> Unload. Then hit the radio button next to com.apple.Finder and disable it.