> you wouldn't be able to execute the next BTC -> USDC leg until after this first transaction to BTC has been "confirmed", and each leg of the trade "can easily take 30 minutes to confirm" all the while exposing the position to BTC downside volatility risk?

Precisely.

> find it very strange that crypto exchanges don't handle execution, confirmation and settlement in the same way as traditional markets

You don’t have to use an exchange to trade crypto. You can send crypto from one wallet to another without any exchange or anyone else in the middle (Except miners who are paid to confirm the transaction)