Edit : the code at the bottom might not be necessary because event max capacity might become available in a not so far future through FEATURE: introduce max attendees for events by SamSaffron · Pull Request #34313 · discourse/discourse · GitHub
Meanwhile you can try this temporary solution which gives you a result like on the picture below. Keep in mind that you need to add somewhere in the title of your event a matching item like 2p, 3p, 4p, or any number up to 500 with p after it will work.(you can change this letter to something else to like 8 spots, 8 max cap etc. by modifying the code as detailed in the solution)
When the “going count” matches the url/topic title then the going button changes to “full” and is clickable anymore unless someone frees a spot.
It has no waiting list though as Javascript can’t handle that, adding the registration timestamp on the participant view is as you say very important(and not too difficult to achieve from a development point of view). If the registration timestamp was available, the interested button could be changed to “add me to waiting list” with CSS. This would at least allow event organizers to add and notify users from the waiting list that they have been added to the event.