Even if we look at what it means to have sovereignty on Lightning, that in some way means having shared custody of a UTXO with a channel partner, so I am not sure if we have any solutions for end users to have monetary sovereignty as Bitcoin scales to 8B people.

The main solution I keep hearing about right now is ecash, which in the best of cases would involve federated custody. Effectively monetary sovereignty for small local banks, but not for the end user.

That said, we often innovate solutions as the problem becomes more immediately painful. As we reach the breaking point where it becomes near impossible for individuals to hold and transact Bitcoin in a sovereign way, unless they have very large amounts, we will naturally have more people with motivation to come up with a new solution.

I don't have any ideas about what that solution will look like, but it will likely be different from what anyone today has conceived, or else we would have it in the works already.