Who says you can't build non-ftl ships, plus technologies for sending things like corvets and frigates through could exist, especially if you double down on the gate network? outside of crisis I don't think intergalactic is a thing as there is only one galaxy and intergalactic means over more then one galaxy. If you are talking about crisis then you can simply make the crisis inable to use them, or let them use them, it is the crisis after all. If you are speaking to declaring war and instant invading, unless you have the total war cases beli then you would still have to win the war or conquer the system and settle for status quo.
As for getting to systems outside the gate network, the answer could be a mix of diplomatic deals, special projects, or even the possibility that you can't if you don't go the ftl route. The game doesn't require you to conquer the galaxy to win, you don't need to be able to reach every system unless you our playing without victory year and at that point victory is defined by whatever you want anyway.
