There is a way to use ess-cycle-assign: edit the variable ess-assign-list and bind a key to ess-cycle-assign.

Here is an example:

(setq ess-assign-list '("<-" "%>%"))
(with-eval-after-load "ess"
  (define-key ess-mode-map (kbd "C-c _") #'ess-cycle-assign))