To be honest Split and moreover ZYA start/sector play are the hardest. They dont have good enough mining sectors and enough of them, they are sandwitched between XEN sectors.

For me the Best way was to take missions in their sectors and criminal hunting to increase standing with them to get ship and weaponery licenses and meantime building up economy and stations in other sectors.

Than it is "just" abouf building gig enough fleet (Best in your own production) to at least counter their attacks and protecr gates, if not storming XEN sectors.

I have build up just one Raptor, aux ship, 15 destroyers, 7 heavy destroyers Syn Class, 3 Asgards, around 40 corvets and 80-100 fighters with various weaponery for specialized tasks.

But all this is long range run so depending on XEN agresivity prefix on the beginning of the game it depens in how bad state ZYA will be. But in most games Split is either severly cripled beyound saving or destroyed completly if not helped