There is great opportunity to continue to make ZCash everything Bitcoin could have been and embracing L2s is part of that IMO.
Agreed. And for us it would honestly be an amazing field of experimentation / anticipation on what Starknet on top of Bitcoin could look like in the future.
am I correct that bridging shielded ZEC to an L2 account would require future work and is outside of the scope of this initial proposal?
Correct. Outside of the scope of this initial proposal. But this proposal is a prerequisite to think about this as a later step.
it seems the main or sole benefit of the L2 would be driving tx fees to the L1 via settlement.
Yes the main advantage is scaling, but not only. Also UX and expressivity. With account abstraction you can build amazing UX, especially for normies, while remaining trustless and self custodial.
IMO it makes sense that any proposal includes at least a plan for privacy on the L2 from the beginning, even if the implementation and rollout is staged.
The proof verification on L1 (this proposal) would TOTALLY enable this going forward. To be specific and give a concrete example, once we had proper ZK support in stwo (our next gen prover) and privacy support in the Starknet OS, this would naturally enable privacy on the L2.