There are only some small reasons to do move away from Eventbrite: not relying on another service and keeping a little more data privacy (Eventbrite knowing about your events… using it for advertising etc.) But I guess that’s probably not enough reason to switch right now if Eventbrite is working and people are already signed up through there.
For the future, a simple solution to do the visitor numbers on the forum could be to use the polls, so people just click to register. There are probably more advanced plugins/methods. I have seen buttons with “register to reveal this content” on other Discourse forums but researching and enabling that seems like a lot more work.
Edit: Though events in the calendar plugin do look interesting