I don't understand what the goal is here.
A good principle when quickly evaluating a position in Diplomacy is to compare how many centers you have with how many units you need to defend those centers. The more "spare" units you have, the more scope for attacking. If you have no attacking power, you end up sitting there waiting to be torn apart when somebody gets around to it.
The key problem that Austria faces in gunboat is that the Balkans are so dense in centers that typically everything except their northern border will be militarized. Thus, a common Austrian problem is that even when not threatened with imminent harm, it costs 4 units to hold 4 centers or 5 units to hold 5 centers (add Greece), which means you end up having to sit there and watch others grow.
So, if you're going to take chances to improve the situation, the goal should be to grow in a direction that lets you add more centers than it costs units to hold them.
If you take Venice, that's certainly not true; unless you're at war with Italy, it costs at most one unit to hold Tri. If you take Venice, it will cost you at least two units to hold it. If you can get to Rome,then often two armies will hold Ven/Rom, so at that point you've gained something. But that's not very likely with this sort of attack, is it?
In the meantime, this leaves you with too few units to hold Vie/Bud/Ser. So, that's always going to be a tempting target, and you never really get to repair the problem, because you're permanently a unit short.
In a press game, you can negotiate your way around this, of course; perhaps R or T will agree to demilitarize to your mutual benefit. But in gunboat? Not likely.
A similar argument should explain why Italy doesn't want to, say, stab for Munich [although it often does in gunboat, hurting both G and I]. You get one build but it costs you a lot more than one unit to keep that build.
What are some good early expansions for Austria? Adding both Gal and Rum (with nothing in Boh) lets you hold a line with as many units as Vie/Bud/Ser, so it adds attacking power. In many cases, adding Bul will free up a unit. For the most part, other expansions will cost units rather than freeing them up, so you'd want to know enough about the other players' intentions to be sure that's a good decision.