The biggest difference that stood out to me was that the fuel compositions are almost exactly opposite; 85/15 ethanol/gasoline for Indy and 10/90 for F1.

I was able to find plenty of articles saying that next year F1 will move to a "100% sustainable fuel", but none that actually mentioned the composition. Is it likely to move closer to the make-up of the Indy fuel?

Every engine supplier has their own fuel supplier contract so the fuels won't be completely identical.