>seed(block) = hash(seed(block.parent) + block.proposer.address) No grind capabilities at all. No matter how many times you try, you will always generate the exact same result. The randomness basically comes from absentee proposers, so the only way to influence the outcome (not proposing) is costly as you sacrifice your reward and all other influence over the block beyond signing / not signing it.

That is trivially grindable. If the seed is your address then you create an additional address such that the next winning address is yours.