But brute force, how much entropy are miners brute forcing into?
There is no exact number.
As already mentioned, the difficulty of the problem is changes every 2016 blocks, so the average block time stay close to 10 minutes.
If the average block time is more than 10 minutes, the problem becomes easier and if the average block is less than 10 minutes, the problem becomes more difficult.
with such powerful hash rate, will it be used to brute force against our private keys? I guess the answer is no.
That's not possible.
Each private key provides 128 bits of entropy.