Not sure if this has been asked, but how do I create multiple AI personas that I can play against?  

I know I can download other people's personas, but I have exactly zero interest in that.  Why would I want to play against a character that some rando on the internet created?  I want to play against my own creations!

I feel like there should be some way to do this, but I can't find a way to do it.  I think it would be a massive oversight if the devs gave us such a unique tool, but didn't let us actually utilize it in any meaningful way.

I like the idea you propose.  They should  allow players to test their own strategy against themself at various levels  of difficulty, that is one way to know if a strategy works or not  to whatever extent based on a Fame score, generally.  Its kind  of like when  a competitive  outdoor sport like soccer does a scrimage.  It provokes good practice, so that when faced under different circumstances you can do well against other single player Ai, or even multiplayer.  Of course,  I am reminded that true human opponents play differently than the computer does always - but still  the test would be fun  to see.  Do you get along with your own empire, or not?  Do they steal your favorite  cultures, and force you to adapt?  I like these thoughts.

I wish that was possible too.  A work a round of sorts is that if you click on the circle icon on the AI picture (when setting up a game), you can change a heck of a lot about it's AI; look and color.

Check the ideas part of this website, I proposed the idea a while back and we've heard nothing from the developer about it. Who knows what will happen.