Where is the mistake in my RSA by-hand calculation? Like several previous askers, I seem to have made a mistake in my RSA calculation, but despite going back over it three times, I cannot spot it.

I picked 1000003 and 6000011 as my primes, p and q.

n = 1000003 * 6000011 = 6000029000033
z = φ(n) = (1000003 - 1) * (6000011 - 1) = 6000022000020
e = 17 (arbitrary pick of small integer that's coprime to z)

Public key is (e, n) = (17, 6000029000033)

Found d by solving (17 * e) mod z = (17 * d) % 6000022000020
d = 857146000003


d was calculated with an online multiplicative inverse calculator. I suspect this is where the error is, but I've redone it several times with different calculators to no avail.

So then I encrypted the number 6:

encryptedMessage = (message)^e mod n
encryptedMessage = (6)^17 mod 6000029000033
encryptedMessage = 4926601444670


Decryption is where I realized something must be wrong, because:

message = (encryptedMessage)^d mod n
message = 4926601444670 ^ 857146000003 mod 6000029000033


Except obviously that exponential operation is too huge to be done. I can't calculate it anywhere. It crashed my interpreter when I put it into python.

What am I doing wrong here?? :(

Or, perhaps, am I doing it right, and it's just that there's a necessary efficient method for this calculation which I'm not aware of?
Your $d$ is indeed incorrect. I get $d = 4235309647073$, using Wolfram alpha.

As to python, use the builtin pow function with a third argument equal to the modulus.

So 


message = 6
encmessage = pow(message, 17, 6000029000033)
assert message == pow(encmessage, 4235309647073, 6000029000033) 

which will apply a smart algorithm like those in answers to this question. Never compute the direct power first!