Correct. Could the grounding busbar in the subpanel be used for this purpose, since it meets the requirements such as being aluminum or copper AND 1/4" by 2" minimum.
the minimum requirement per NEC is 1/4" thick by 2" wide. the grounding bar in the subpanel is not 2" wide.
If you can, just install a bar outside of the panel and connect all of your existing GEC to it.
