I'm not sure how biased an average die is
Exactly the point. If you have no idea how biased your dice are, then why would you feel comfortable using them to generate something as sensitive as a bitcoin private key or seed phrase? That's just irresponsible.
maybe it is. but i think there's worse things someone could do to generate a private key than rolling a dice. like using a computer connected to the internet and generating it right off a live website such as bitaddress. how many people have been hacked that used a private key generated by rolling some dice? haven't heard of that happening...
not every bitcoin private key has exactly 2 hex characters of each digit...so for most private keys there is going to be one hex character at least one that appears more than the others. whether that came about through a biased dice or a random number generator on a computer, you would have no way of knowing.
A more practical approach would be to simply use the von Neumann approach I alluded to above. Take any coin and flip it in twice. If the first flip is heads and the second flip is tails, write down 0. If the first flip is tails and the second flip is heads, write down 1. If the two flips are both heads or both tails, don't write down anything. Repeat until you have 128 zeros or ones written down. This method completely eliminates any bias in the coin and produces a uniformly distributed output. It will require a lot less flips than any method to test whether or not your coin is actually fair.
never heard of that method but after analyzing it, I guess it does work since the probability of TH and HT are equal. Which is all you're counting. When you get HH or TT, you ignore it. maybe that same method could be applied to rolling a single die but it's not clear how.