A question is how much entropy a string like this has? Say you have 10 million English songs, if you always start a password with the first letter from a song,

A question is how much entropy a string like this has? Say you have 10 million English songs, if you always start a password with the first letter from a song, and choose the password to have between 20-30 letters, you only have about 26 bits of entropy in the password, which is trivial to brute force. You can increase the entropy by starting from a different word, or inserting random characters, but the entropy will never be close to the entropy of a random string of the same length.