This is far less interesting than other "plus the number of cards named ~ in all graveyards" cards, because the jump from 6 to 7 is so minuscule.

Yeah. Might be worth being "X is 6 plus twice the number of", although that also breaks the cycle

Also, mill is a keyword now

Hmm; maybe? It's a small bump, but assuming you do get all four uses of this, it's 6 cards - which is both pleasing symmetry and 10% of a deck. I agree it's not huge, but it's not chump change either.

That's quite a big assumption though. I have a deck that does usually get all 4 Kindles out, but only because it contains loads of Teferi's Puzzle Box and Mindmoil. Typically the Strength Burst-type cards you can only count on getting 1 or 2 of them.