Does Coinbase support withdrawal through BSC network? If so, you can do it that way. You would need both BNB and USDT (BEP-20) to buy on BSC AMMs/DEXs though.

Just an FYI that in most platforms, USDT are converted back to BNB or WBNB before it's swapped to any BEP-20 token. As a result, the transaction fees could be higher compared to buying with BNB directly. Good thing the added fee is minimal on BSC.