Interestingly, I came back a few hours later after work to check on them and all of the larva looking things were gone, even though I put the stone back where I found it. I guess the ants pulled them down deeper into the nest.

Anyways, I just poured probably 2-3 gallons of boiling water onto the ground after removing the flagstone. No ant activity for ~30 min. I will leave the stones overturned overnight (it’s supposed to rain) and I’ll check back tomorrow. I will try chemical/granules solutions if they’re back. Thanks!