Creatures with toxic deal poison damage each time they attack
Simisear is an unlikely choice for a pair of Ultra Rares in the final Sword & Shield special set, but hey why not, right
Otherwise, start with ST-01 or ST-28
This 3 mana 1/4 isn't just a decent blocker - it's also a great way to deal with cards causing trouble in your opponents' yards
I never find myself in a position where I want to take a turn off to cast it, and would much rather play a more niche effect like Shimmer Myr (cheaper, able to be flashed in and easier to recur) or Vernal Equinox, a card I mentioned last week when I talked about optimizing for fun
Even for a relatively simple and un-interactive effect like self-mill, there are a dizzying number of ways to achieve it and all sorts of secondary considerations which might sway you towards one solution over another