Yep, they're the only ones in the main. I'm trying to keep wincons low with the idea that I'll try to get a lock first and worry about a win second. It's possible that more threats are needed.
Ballista comes out of the board in a lot of matchups, often just as another way to win. The deck often goes to time, and having a threat that can end the game fairly quickly helps. Ballista helps a lot vs dredge by enabling a win with a Bridge on the table. Once a true lock is established, you can use Smokestack to clear things like null rod to turn ballista on.
I'll say that the deck is far from optimized. It was really meant as a proof of concept to see how good Ghirapur Orrery can be. The biggest problem is Planeswalkers. For that reason I think cutting a Revoker wasn't the best idea. If anything a fourth would be great.
Untimely, I think Orrery has promise but I'm not sure how legitimately competitive it can be. My hunch is that it isn't truly competitive, just good enough to earn back playpoints.
But I will say one thing: Orrery is a blast. Some of the games are hilarious, and that's worth a lot to me.