Countably infinite does NOT mean finite. If you shuffled an infinite number of Clown tokens into your library, you would never draw another kind of card in that game. Infinity plus however many cards were in your library is still infinity. Imagine if you didn't shuffle. How many cards would be on top of the first real card? Infinite cards are on top, so you'd never draw the card. If you shuffled the deck, that would still be the case. There would be an infinite number of Clown tokens before and after each real card, because the chance of getting a Clown token between any two cards while shuffling is 100%. Not 99%, not 99.999...%, just 100%. Infinity divided by the cards in your library is infinity. You can never draw any real cards, because even if you took off the top card, infinity minus one is still infinity.Grenzo, Dungeon Warden would always hit, since there are an infinite number of Clown creature token cards on the bottom of your library. Since tokens have 0 mana value, you can't cascade into any real cards. You could mill out if you got hit by Tasha's Hideous Laughter. If you chose Land for Abundant Harvest, you would get one if you had a land in your library. However, if you got hit by Cut Your Losses, you would only mill Clown tokens, and no cards from your library. Infinity plus the number of real cards in your library divided in half is infinity, so you'd never get to the first real card, and you'd still have an infinite number of Clown tokens on top. Finally, if you happened to play Braingeyser with X equal to infinity, you would STILL not draw any real cards. Infinity minus infinity is infinity. You would have an infinite number of Clown tokens in your hand, and an infinite number of Clown tokens on top of your library.It's easier if you think of infinity as a description or a quality rather than a number. Treating it like a number infects whatever math you're doing with odd-looking results.

FYI

Is Abundant Harvest also in the main Strixhaven set or is it in a future set that hasn't been released/announced yet?

Abundant Harvest is a “preprint” from Modern Horizons 2. It does not appear in Strixhaven outside of the Mystical Archive.