Don't forget to take into account that every time you complete a dungeon-thing, it increases in difficulty (until after the 3rd time). I suggest making sure you avoid some dungeons until your very last character gets a chance to play and get an artifact. (In other words, save the easy passes for the beginner characters.)

I suppose you could do three characters per year, or two, or whatever you see fit. If it were me, I'd switch off between characters after every stage completion just to keep things interesting. :D

Be prepared for a long time spent on you Gamecube though. You WILL be powering up 8 characters instead of 1, you know. :3