I'm going to be honest, and say YTA. Especially by how you're going on in the comments.
On one hand, you're saying:
> I told my GF that this year I would be more flexible, and have more availability to do stuff and plan travelling. And she was happy to hear that, and understood.
That's right, she can be supportive of that, and happy to hear that aswell. That means you won't be stressed and worked to the bone as much.
Then in the same sentence you're saying:

> My GF asked about travelling times, and I told her that she can't expect me to know availability for June & September, because that's so many months out, and I can't guarantee certainty. And she understood that.

You already promised her going into 2022, that you could make travel plans and go places with her. Work got busy (understandable, life happens), and you couldn't go. This year, 2023, she still wants to travel. After putting HER life on hold, for YOUR work, she doesn't want to do that again.

You told her you can't commit to June & Sept upfront, so she booked around those times. You told her you could still travel in general? Great, do it when you can. But you can't expect her to not have any independence or freedom and put her life on hold, AGAIN, and have her dreams pass her by after they have already.

And honestly, dude. Start up company or not. If you can't tell your employer or company.. "Hey, I have a prior obligation and committment on this day, don't schedule me around this time. I'm going on a vacation." and would rather put work ahead of your own girlfriends happiness and spending time together, then yeah. That makes you even more of an asshole, and you'll never get any free time.