Got my procedural ratings 19 years ago and still work a procedural sector now.
Rate of climb is a legit form of separation and I continue to use it today (when appropriate).
As mentioned above, you just need to establish initial vertical separation and then bob's your uncle.
Care should be taken to ensure that the rate of climb can be maintained to the required altitude where another form of separation can exist, as most A/C ROC significantly decreases with altitude, I would not assign this form of sep above about FL250.