There’s also the notion that by the time we are traveling far enough that maintaining consensus is difficult, my bet is that the on-chain hardly moves other than for large transfers. Tx rate will not be an issue. L2 and above will continue to be a ledger based on the underlying chain (and its more concrete nature). 

This seems totally doable. 

This also assumes no advancement in communications if we are traveling that far. 

I think something like an ansible is going to be possible *before* FTL is possible. For one, if we can manipulate gravity, we can modulate it. And we’re already listening to it (waves from black holes traveling near the speed of light). Gravitational waves have no speed limit.