What’s been the most stable written language over time?

The best possible combo of far back and wide reach is probably Akkadian cuneiform. Maybe you could get further back with Sumerian cuneiform and trust that it’ll get translated once Akkadian becomes the lingua franca.

Oracle bone script which becomes Chinese characters probably doesn’t get you as far back and maybe not as wide an area, but definitely much longer in time since it’s basically still in use.

The only problem is those are both languages of imperial systems and giving them greater power is probably a net loss for humanity.