Is it possible to send mail using Command + Enter in Mail?
You can look at this answer and apply accordingly to your case.
TL;DR version:
I tried to use the System Preferences, Keyboard Shortcuts tab to define Command+Return to send a mail. But the effect was that Command+Shift+Return sends mail.
I found a solution in Mathias's dotfiles. In a Terminal window, type:
defaults write com.apple.mail NSUserKeyEquivalents -dict-add "Send" "@\U21a9"
Then restart Mail.app