*Does using a passphrase really make the wallet 100% secure?
It depends.
A wallet with only the mnemonic recovery words required for a full restore is as safe as you can keep the mnemonic recovery words backup safe. If some entity finds your mnemonic recovery words, the wallet usually can be restored and your coins can be drained away.
If you extend the mnemonic recovery words with a mnemonic passphrase, then both parts are required to restore the wallet. Obviously it doesn't make sense to store the recovery words and the mnemonic passphrase together where they could be found together.
If you store them securely and separately now if an entity finds your mnemonic recovery words, your wallet remains safe because the evil entity can't restore your wallet without the mnemonic passphrase.
*Trezor vs. other hardware wallets—which one is more secure and why?
I would argue that most if not any hardware wallet that is open-source, has preferably reproducible firmware builds and documented hardware components and documented attack mitigation can give you confidence that you know what this wallet does under the hood, if you're able to read and understand it.
A good and open communication of the hardware wallet's company is an additional bonus and in my opinion important, too.
Companies with some years of experience in the field are also preferable over the newest shit. Long-time experience is beneficial.
So, Ledger crap with its closed-source firmware shit is out of discussion.

I would also say, that hardware wallets with Bluetooth are less secure, because Bluetooth hardware and software stack are complex and error-prone and just open another attack vector.
*What are the benefits and risks of using a hidden wallet?
Benefits:
A sacrificial and "canary" wallet which can be restored just by the recovery words can indicate a compromise of the recovery words.
Any unique mnemonic passphrase generates an unique wallet. So with one set of mnemonic recovery words you can have as many unique wallets as you use unique mnemonic passphrases.
Risks:
You have to carefully and securely store any mnemonic passphrase separately from your recovery words. There is no error tolerance. The smallest error in your mnemonic passphrase gives you a different and empty wallet.
If you loose all copies of your mnemonic passphrase, your wallet is lost even when you still have the mnemonic recovery words. Because if you had a complex enough mnemonic passphrase, it's not likely you can brute-force it. Well, it shouldn't be feasible to brute-force it.