5e d&d has rules for this in the Dungeon Master's Guide, but even then the edges between different CRs are fuzzy. Status effects in particular are a pain to quantify.

If you're throwing only three or fewer zombies at the party, you could maybe justify CR 1/2, but if there's a bunch of them CR 1 seems more accurate to reflect party members getting regularly stunned.

Although for playability I would maybe change Scream from "(Recharge 5-6)" to "(Recharge 6)," just to reduce the odds of stunlocking the party to death.