I'm torn: should I leave the second ability as it is (, : Exile all cards from all graveyards. Put Seal of Rassilon on the bottom of your library.) or change it so that it's a reprint of Relic of Progenitus?

Not much odds in it. I might say that there's not much point doing the tweak unless you have a reason to want to. The tweaked version is potentially unlimited protection against mill, but doesn't have the important cantrip which makes it much less playable normally.