Where would you make make the cut that takes advantage of object store parallelism?

That is, at what layer of the stack do you start migrating some stuff to the new strongly consistent system on the live service?

You can't really do it on a per-bucket basis, since existing buckets already have data in the old system.

You can't do it at the key-prefix level for the same reason.

Can't do both systems in parallel and try the new one and fall back to the old one if the key isn't in it, because opens up violations of the consistency rules you're trying to add.

Seems trickier than one might think.