Quote from: lyw123 on Today at 02:14:59 AM

(1) If I set a passphrase with at least 15 characters (0-9, a-z, A-Z), then Trezor or bitbox will never know it and cannot crack it? This can prevent hidden backdoors in the mnemonic seed, which is very difficult to verify.

If there is a backdoor why would you think a passphrase would solve it?

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(2) If I wait a few months (>2 months) to update the firmware after it was released. Then this can against malicious firmware from Trezor or bitbox company. (see “How to prevent if The Trezor release new firmware update to steal Bitcoin”, https://bitcointalk.org/index.php?topic=5386771.0).

If there is high-risk vulnerability being fixed you should not wait to update. So it depends on what is being updated or fixed.

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Are there any other possible risks from the hardware wallet companies?

Supply chain attack on the shipping process and their mitigation to prevent that. Data leaks.

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Is it really necessary to buy a bitbox for spread risk? Of course, I will not use ledger for altcoins. Half in trezor, and half in bitbox.

Spreading your funds into separate HW surely mitigates the risk by any means, but the question is can you stand with the complexities? I rather think you add unnecessary burden, noting you already have 6 HW.

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Is it necessary to buy a new computer (~100 $), which is only used for send cryptocurrency?

I don't think so. HW should prevent attacks from the computer itself. Furthermore, you might want to consider about cold storage option if eventually you want to buy a new computer, so you are not relying on HW.