I've organized quite a few cruises, and have come to the same conclusion you have. Too many cars ruins the experience, and creates issues. I no longer have public invites to a cruise I organize. While this limits the exposure to new members, and can cause me to miss out on developing a new friendship, I have found that I feel much more comfortable going on cruises with people I know can be responsible. Typically, this number tends to be in the 3-10 car total, range. The list of people I include has exceeded that, so I typically will mix it up on the invite, based on location (there's a wide area that I typically cruise in, and can easily get a group of 5+ people together within an hour).
My advice is to keep your list of attendees small. Limit it to a group of people who will appreciate it, and will act in a responsible manner. If other people choose to make their own up, let them be the responsible party. However, with keeping the same spot, people will show up there expecting to just be "let in" since they're already there. If you go the invite-only route, you'll have to change the meeting venue.