Thanks for the tip, I'm aware of it, but it breaks the core benefit of Alt+X : guaranteed repeats, with fn-control-f2-F-Enter you can't be certain to open the File menu since there might be another menu starting with F

Predictability is a core feature of UI design

Anyway, personally I use a better alternative to fully recreate the Alt+X to open a menu via keyboard maestro, so I've sovled the issue for myself, but it still is a core Mac deficiency contrary to the "OSX has been very keyboard friendly" claim