external stack will make your app very slow. (An external RAM access takes between 5 and 10 times as long as an internal one.) Can you take that hit? If you can, that is an option.

In SDCC documentation there is a section on this. Some things, like using bit variables, can cause RAM to be wasted, because the stack starts higher up than usual. Also there are compiler options that may help.

Other than that, all you can do is go through your code and see if you can reduce RAM usage.