GTVA possibly managed to live longer than most species by surviving the Lucifer armada (and although there are 2 instead of 1 species) but still they expanded to only 30 systems; I doubt that other species (except the Ancients) could get their hands on significantly more systems before being discovered and eliminated by the Shivans.
You are canonically wrong. The Ancients expanded into other galaxies before meeting the Shivans. The_E explained above why the Shivans might have reacted more quickly to the Terran/Vasudan conflict than they did to the Ancients...and yet canon even leaves open the possibility that the Shivans reacted faster
to the Ancients than they did to us. (I don't believe it, but it's there - the suggestion that the Shivans attacked the Ancients within months of their first xenocide, whereas they took at least 14 years with the Terrans and Vasudans).
e: Okay rereading this I see 'except the Ancients', lol. I guess it then depends on what you think attracts Shivans - whether it's subspace activity or interspecies conflict.
The notion that the Ancients explored 100 billion stars and covered an expanse 150,000 ly in diameter on sublight drives is so ludicrous it can't be anything but hyperbole. By the time a ship got from one end to the other, they might not even be the same species anymore.
It's not ludicrous at all. It simply requires a psychology and culture very different from humanity's. Thousands or millions of years of internal conflict between branching sects of Ancient descendants could produce the instinctive, competitive, universal xenophobia the Ancients exhibited. They were 'a proud race, and always the strongest'; once one subgroup of Ancients got subspace, they would have such a total advantage over the rest that they could probably rule by sheer threat. And their history of internal competition would make their instant, xenocidal expansion natural and normal for them.
Not if you take relativistic time dilation into account... but if they had become too good at sublight missiles they simply would bombard the Shivans with relativistic missiles.
If you take a radius of 5-10 lightyears (Alpha Centauri is 4.37 ly from Earth) the number of systems is highly limited and it takes a long time to reach them but you still can stay in contact with them.
The Ancients primarily using relativistic weapons is exactly why I believe the Shivans destroyed them. The Lucifer was immune to relativistic impacts due to its shields, and the smaller Shivan vessels could simply jump out of the way - or into the local reference frame of Ancient ships, to tear them apart at ranges so short the Ancients would never have even considered them viable for warfare.