Posting a Reel to your Stories will help a bit with reach. Usually, for most people, your feed posts get a lot more reach than your Stories, and this is because there's a ton of Stories inventory, so there's a lot more competition relative to feed. But it will just generally be additive, so it's a good practice, as long as it's not something that annoys your followers.
In general, the ranking system [for Collab posts] biases more towards the original collaborator than the person who accepts. It really shouldn't matter, but right now it matters a bit, so if the original collaborator is the larger account, that will help on the margins.
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