@bsammon I don't see performances as being inherently different from other events, just on a smaller scale, so I'd advocate for just creating a new subtype. We already have "part of" relationships with perfectly workable language, and I'm not sure if we'll need many changes beyond a new icon and support (on both sides) for a new entry in the type list.
The biggest question is whether individual artist performances need to be separated out in the first place, and I think that's where "that's not how it's thought about in the real world" comes into play and where the largest difference between concerts and festivals lies. With the former, the acts are all conceived of as a single block and would be most accurately represented with a single setlist. With the latter, the acts are listed individually with their own start and end times, and are a very good fit for separate events. Which one outweighs the other is at the core of this disagreement, but my take is that it's easier to emulate the former with the latter, and conceptually cleaner than trying to separate a list of twenty links (likely intermingled) between five groups of performers, no matter how clearly they indicate themselves.
I'm thinking of the last paragraph of @ChurruKa's proposal:
I think we should allow the guideline to be "relaxed", i.e. it is allowed to add each artist to a single event that spans the whole festival, add all artists from a single day to per-day events, or adding supporing artists to the main event instead of creating individual events etc but as a way to ease editing and keeping it clear that the correct way is to add data as fine-grained as possible
It's like works: with a few rare exceptions, recordings are encouraged to link to what they're a recording of. For ease of editing, we don't require it
and given how many people blindly duplicate recordings, that's probably a good thing. Maybe we even encourage different formatting for concerts than we do for festivals. Either way, just because one is "better" doesn't mean the other's "bad".
It would help, though. We have two individual performances scheduled for the same time. One of them's displayed as being canceled. If we want to, we use the "rescheduled as" relationship (or a new, similar one) to link the two without going through the enclosing event. Does require the times to be displayed in the "parts" list (and admittedly works best if "parts" and "canceled parts" were listed alongside each other, unlike how they'd actually be separated), but it's easily possible to represent purely in the database without relying on notes in the annotation and/or setlist.
Finally, to add a couple more edge cases to what needs to be considered for the formal guidelines, what about recitals or other events ("concert" fits well enough) where many artists play one or two songs each, or less formal jam sessions where a likely-limited number of people rotate on and off stage without regard to larger groups? Both probably cases where, even if we settle on separate performance events, we encourage joining them anyway, but we'd want to consider them however this turns out.