(defun my-comint-send-input-maybe ()
"Only `comint-send-input' when point is after the latest prompt.
Otherwise move to the end of the buffer."
(interactive)
(let ((proc (get-buffer-process (current-buffer))))
(if (and proc (>= (point) (marker-position (process-mark proc))))
(comint-send-input)
(goto-char (point-max)))))
(with-eval-after-load "comint"
(define-key shell-mode-map [remap comint-send-input] 'my-comint-send-input-maybe))
Maybe consider prompting on a multiline send, since that's not usual.
https://stackoverflow.com/a/52212547: