I can't think of any reason why Command ⌘ + Enter ↩ would not send a mail draft, instead of the current shortcut, which is Command ⌘ + Shift ⇧ + D Is there a way to change this?
You can look at this answer and apply accordingly to your case.
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