There was a time many years ago when I wrote three different stories with three different fellow writers.
Nothing publishable came out of it, but it was a great experience and it tought me a lot.
I think your first point is the one that worked best for me. It’s easy to think that what we’re doing is the best possible path. Sometimes we think it is the ONLY possible path.
Working with another writer soon teaches you that there are innumerable paths you can take and the one you are following might actually not be one of the best. It sounds like an easy lesson. It isn’t.