The Dance of Eternity is quite a challenge yes, but it's not nearly their hardest song to play; I can't do the most difficult ones (the solos are the main problem
)
Yeah, you'll need 7 strings for Dance as well, though their are versions floating around for 6 string. Quite a lot in Seasons could be played on six strings, but then you're missing out on some of the more awesome riffs.
And Afterlife is fucking difficult man, I don't have the strength to continue the picking throughout the whole song
I've done Ytse Jam, but the other two you're mentioning I've never tried. If you ever want to try something something wacky, try Another Won. It's way more difficult than it sounds