I feel this would work best as a static ability:

• "As long as there are less than a total of ten creature cards in all graveyards, ~ isn't a creature."

Otherwise Arixmethes-tech for Svogthos makes a lot of sense.

Good point. I was trying to follow the Arixmethes template, but there wasn't enough room for the counter tech and also the P/T setting and mill abilities I really wanted

I actually just noticed that this isn't a land and just removing creature as a type would make this typeless, so you want "is a land" rather than "isn't a creature". I took my wording from the Gods that are enchantment creatures.

How important is it to you that this mills on attacking? Once it's a creature you don't need milling as much, so you could just make the mill effect part of the mana ability:

• ": Add or . When you do, each player mills a card."

Ought to save you a line of text.

I really liked how the triggered ability plays differently when this is a land versus a creature, so I'd rather not lose it. Having a way to mill is the least important part of the card, though