I'm so confused by what role a CBDC is even supposed to play... why would the government establish a new currency pegged 1:1 to its existing currency? Why not just... allow normal Americans to keep deposits at federal reserve banks?

CBDCs are programmable: whoever controls the CBDC can decide where it is or isn't spent.

Well they could, but actual implementation would be via an intermediary commercial bank that held it's deposits with the Fed. Central banks don't want to have to deal with retail customers. But the result is the same.