The only loose end for me....why was Rio trying to kill Agatha?
Consider:
1) Rio is literally Death, the cosmic entity.
2) Rio does, in fact, love Agatha, and wants Agatha to be with her.
So, it isn't so much that Rio wants to kill Agatha, but that Rio wants Agatha to die and be with her. If Rio just
kills Agatha, what kind of relationship would that be going forward?
I assume Agatha has lived past her time
I don't assume that. These witches living super-long times is a thing, part of how the universe works. Lilia and Jennifer were very old - even with powers bound for a century, Jennifer wasn't aging.
My thought being....perhaps it was the Darkhold itself keeping Rio away from Agatha, and that was the loss that brought Rio back into the picture, but its never made clear.
So, as above, Rio can't just take Agatha. For emotional reasons, Rio needs Agatha to
choose to be with her. Or to come to her by some other agency.
When Wanda's spell finally breaks, Rio shows up because 1) Billy is there, and Rio has an interest in Billy, as he ought to be dead. And 2) the Salem Seven are coming for Agatha - getting on Agatha's good side when Agatha is about to die is a good idea.