dangit, this is really good and simple: wish I thought of this!
It is a cool idea, but I would instead make it so you just draw 6 cards for every hand.
it was the idea, but i didn't know how to write it "correctly". Rules says you draw 5 cards at the en of your Clean Up, so the first hand is an special case making this too wordly
I had also considered the same idea, with Last Footnote's version, except I had "Each player starts with 8 Coppers and 4 Estates. When you draw a new hand, draw 6 cards". I think "draw a new hand" should apply to turn 1 as well
EDIT: Not sure whether my version would work with Shelters and Heirlooms. I think you could make the argument that, based on the usual principle of arranging set-ups so that all would work, you'd have no issue with Heirlooms. Heirlooms' rule says to replace a starting Copper with the Heirloom, so you just replace one of 8 instead of one in 7, but Shelters says to replace "the starting Estates" with the Shelters, so I think a Shelters game would technically, with my wording, replace the 4 Estates with the 3 Shelters, so I think your "add a Copper and an Estate" rule works better in Shelters game - you'll clearly start with 3 Shelters and 1 Estate that way