I see that you’ve used a Dropdown field, which is one of 2 possible workarounds. The 2nd solution is to use a Number Scale field, where it’s possible to set a range, but I think your solution is much better:
Currently it’s impossible to show a custom message instead of the form if the limit is reached. However, we have a similar request for the Event Calendar widget - Register right in the widget.
Users suggested adding a Register button to the Event Calendar, which opens a form with custom fields and where it’s possible to set a max number of attendees. So, I guess this idea meets your case best.
Feel free to upvote it and if it gets more votes, we’ll try to consider it in the future updates 