How much entropy does BIP39 have?
How long would brute forcing take?
Can a 2nd factor be included on a wallet, to lock, unlock it?
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I'd take a look at this answer
It seems a very well resourced attacker would be able to find a used private key for 128bit seed. This would be an attack on all keys in the network, not yours specifically. For 256bit seed, it should be infeasible.
You should know that if you generate a wallet with the help of cardano-wallet utility you will be proposed two options: general-factor 24-word seed (2048²⁴ or 2²⁶⁴ possible combinations) and optional second-factor 12-word seed (2048¹² or 2¹³² possible combinations).
That's a real nightmare for a hacker because it's practically impossible to brute force it.