If you consider it important, you have to store it in a real database. No buts. If you don't consider it important, sharded redis works fine.
Just because it works for your use case right now doesn’t mean there isn’t room for improvements to support others too.
Oh good, then you don't need to do any of the stuff that you suggested to do
No, redis is a memory cache, with some ACID like features bolt on.
Even real databases have hard time maintaining consistency across nodes. CAP is a real constraint.