How do I retrieve my wallet balance with HWI?

You can’t.

HWI is stateless. There is no way to query the blockchain and it does not store any information about hardware wallets, so hardware wallet balances cannot be known or retrieved. Furthermore, hardware wallets are inherently offline devices and have no way to know or retrieve your balance.

Software wallets should be used in conjunction with hardware wallets, as software wallets can monitor addresses in hardware wallets, learn about UTXOs, and create transactions.